数字经济时代，平台企业以惊人的速度快速发展，平台型企业迅速发展与成长的经济学逻辑成为本文研究的主要内容，本文认为平台企业得以快速扩张的主要原因在于平台扩边策略与用户锁定策略的实施。本文首先回顾了当前关于平台经济的相关研究，并对平台商业模式、多边市场理论、平台竞争策略和用户锁定策略等内容进行了文献研究，在此基础上，对平台扩边策略与用户锁定策略进行了细致研究与分析。平台扩边即平台通过引入新的业务线，吸引新的一边用户的加入，使平台从双边市场模式发展为多边市场模式。一方面，本文研究了平台扩边策略的实践，分析了平台扩边的基础与策略选择，发现平台不应盲目扩边，而是应先积累起一定的用户优势与技术优势，选择与平台原有业务相关联的领域进行扩边，围绕一条主线搭建平台生态系统。另一方面，通过构建利润函数模型，求解出了垄断平台扩边的条件，并进一步构建理论模型刻画了在垄断竞争市场中，用户完全单归属和部分单归属两种情形下，平台扩边对于平台定价的影响。结果显示，平台引入新的一边，增强了跨边网络外部性，使平台倾向于提高对原有用户的定价，没有实行扩边策略的平台则倾向于降低对原有用户的定价以留住更多用户。用户锁定对于平台而言意味着平台采取手段增强用户黏性，减少用户流失，提高用户在平台上的交易频率。本文通过观察现实中的平台实践，发现用户锁定常用的策略为社交策略与会员策略。本文对社交机制的接入模式及产生的价值进行了研究，同时对58转转平台超过1亿条交易数据进行了经验分析，结果显示社交关系与平台交易具有显著的相关性，即社交机制的引入能够提升平台交易成功的概率，增强用户与平台的互动，进而锁定用户。此外，我们对会员制模式进行了分析研究，发现平台主要向会员提供产品和服务两方面的优惠，平台可通过会员机制筛选高价值客户，使平台集中有限资源挖掘存量客户的潜在价值。本文还对会员制模式进行了案例研究，梳理了几大平台的会员制模式，发现会员制的推行有助于提高用户的忠诚度，提高用户在平台的交易频次。基于以上的研究与分析，我们认为平台应实行扩边策略来扩大用户规模，并实行社交策略与会员策略来锁定用户，提高用户在平台的交易频率，实现平台的扩张与发展。

In the era of digital economy, platform enterprises have developed rapidly at a staggering rate. Premised on this background, we shed light on the economic logic of galloping development and growth of platform enterprises in this work. It is found that the fast expansion of platform enterprises is ascribed to the implementation of platform enlargement tactics and user lock-in strategy. The current research on the platform economy is reviewed and summarized in detail. After carefully analyzing platform business model, multi-sided market theory, platform competition strategy, and user lock-in strategy, we focused on platform enlargement and user lock-in strategy to explore more about platform expansion.Platform enlargement is a business behavior for the platform to attract new users by introducing new business lines so that the platform can transform from two-sided market model to multi-sided market model. The practice of platform enlargement strategy, analysed the foundation and different strategy choice is studied in this work and we found that the platform should not blindly increase business sides. Instead, platform enterprises should first accumulate user advantages and technological advantages, and select the areas associated with core business of the platform to build platform ecosystem. With that, the profit function model was built and analyzed the conditions for the enlargement of the monopoly platform. Following that, the impact of enlargement on the platform in the monopolistic competition market was further investigated under the conditions of user single-homing and multi-homing. The results showed that the introduced new side of the platform could increase the cross-side network externalities. Therefore, the platform tended to raise the price of the original users, while the other platform tended to lower the price to attract more users.User lock-in strategy means that the platform takes measures to enhance user viscidity, reduce user loss and increase the transaction frequency. By observing the platform practice in reality, we found that the commonly used strategies of user lock-in were social strategy and membership strategy. In this paper, we analysed the access way of social mechanism and its value. This work conducted empirical analysis using over 100 million transaction data of Zhuanzhuan platform. It is illustrated that there was a significant correlation between social relations and platform transactions. That is, the introduction of social mechanisms could improve the probability of successful transactions and prompt the interaction between users and platforms. Besides, we analysed the membership and found that platform would provide both product discount and better services charging customers membership fee. The platform could also distinguish high-value customers through membership mechanism, so that the platform could mine the potential value of existing user with limited resources. In addition, we also performed a case study of membership and found that the implementation of membership system helped to improve the loyalty of users and to increase the transaction frequency on the platform.Based on the above research and analysis, we provide specific suggestions for platform enlargement and user lock-in. We aim to help platform enterprises rapidly expand the scale of users and further enhance user stickiness to the platform. Motived by our purpose, this work may help platform enterprises improve the frequency of user transactions and achieve platform expansion.