Focusing on the trade distortion and welfare loss caused by State trading in China’s grain import,this paper undertakes an empirical analysis to assess the trade distortion and social welfare loss of China’s wheat,rice and corn imports with import and domestic price data (1992-2017),by constructing a State trading tariff equivalent model in view of the calculation theory of the trade distortion effect under imperfect competition market.It is found that:(1) The trade distortion and welfare loss could be greatly corrected and eliminated by pursuing profit-maximization operation goals; (2)The administrative TRQ allocation method causes different monopolistic levels,which leads to the most serious distortion degree and welfare loss in the wheat market,less in the corn market and the least in the rice market.Therefore,this paper proposes to lower the level of trade distortion and improve social welfare from the perspectives of optimizing the allocation of TRQ and deepening market-oriented reforms of state trading corporations.