随着2001年中国加入WTO，中国向美国出口增速明显提高，同时中国出口面临的关税政策不确定性大幅下降，关税政策不确定性指企业未来面临的关税变动。关税不确定性降低是否促进了贸易增长？关税不确定性影响企业出口决策的微观机制是什么？基于2000-2003年中国海关进出口企业数据库超过80万条中国向美国出口观测值的回归分析，本文得出如下三条结论。首先，中国向美国出口的增长主要表现为扩展边际的增长，且关税政策不确定性主要通过影响扩展边际影响贸易增长。集约边际指的是现有出口关系出口金额的增长，扩展边际指的是新的出口关系（包括新的产品、新的企业、新的目的地）的建立，中美双边贸易中，新的产品、新的企业带来的贸易增长对贸易总量的贡献较大。关税政策不确定性通过推迟企业支付出口固定成本，对包括新的产品、新的企业在内的扩展边际产生显著影响。其次，关税政策不确定性下降一方面促进企业的出口进入，另一方面挤出部分低生产率出口企业。关税政策不确定性降低通过放松出口利润约束条件促进部分生产率较高的企业实现出口进入。随着更多企业进入出口市场，竞争加剧导致低生产率的原有出口企业退出出口市场。关税政策不确定性对出口扩展边际具有双向效应，既具有“出口进入促进效应”，也具有“出口挤出效应”。最后，关税政策不确定性下降的出口进入促进效应具有异质性。在外部融资能力较强的行业，关税政策不确定性下降的出口进入促进效应更显著。融资约束的存在扭曲了关税政策不确定性与企业出口状态的关系，消除融资约束能强化关税政策不确定性下降的出口进入促进效应。本文研究了关税政策不确定性对出口二元边际的差异化影响，分析了关税政策不确定性对出口扩展边际的双向效应，并将融资约束因素纳入关税政策不确定性影响企业出口决策的分析框架，指出消除企业融资约束能够强化关税政策不确定性下降的出口进入促进效应。本文认为，随着中国不断签订双边多边贸易协定消除关税政策不确定，发展金融市场深度广度、缓解企业融资约束的重要性会日渐提升。

Along with China’s WTO accession, two stylized facts stand out -- an accelerated growth of China’s export value to the U.S. and a dramatic decline in tariff policy uncertainty (TPU) for Chinese exporters. Does TPU reduction help boost the export value growth? What is the underlying mechanism that TPU affects firms’ export decisions? Using over 800 thousand observations of firm-level Chinese exports to the U.S. from 2000 to 2003, collected from China’s General Administration of Customs, we are able to identify three findings. First, export growth on the extensive margin explains a larger portion of China’s rapid export value growth to the U.S.. TPU reductions boost export value growth mainly through its significant positive effect on the extensive margin. Here, we define intensive margin as the export value growth of the existing trading relations, and extensive margin as the export value growth of new established trading relations. In the China-U.S bilateral trading relations, trade value growth from new products and new firms contribute larger to the total export value growth. The existence of TPU, with irreversible fixed cost, delays firms’ investment and export entry, which corresponds to extensive margin.Second, a reduction in TPU not only accelerates certain firms’ export entry, but also expels certain firms out. A TPU reduction, through easing firms’ profit condition to export market, helps firms’ with high TFP levels to enter the export market. And with more competitors in the export market, some firms with relatively low TFP levels will be drove outThird, there exists considerable heterogeneity in TPU reductions’ effects on accelerating firms’ export entry. TPU reductions’ accelerating effect is more significant in firms with better access to external finance. The existence of financial constraints distorts the relationship between TPU reductions and export entry decisions, and eliminating financial constraints strengthens TPU reductions’ effect on accelerating firms’ export entry.We study the different effects TPU on extensive and intensive margins of trade, analyze the coexistence of accelerating certain firms’ entry and driving certain firms out, take into consideration the financial constraint factor, and find the effect of eliminating financial constraints on TPU reductions’ boosting export entry.We believe that with China signing more bilateral and multilateral trade agreements, it will be increasingly important to build a more developed financial market in order to serve the exporting firms’ needs.