The efficient market hypothesis is an important basis of financial economics. The simple statement of the market efficiency hypothesis is that security prices fully reflect all available information. Usually, market efficiency is divided into three categories: (1)weak form tests (How well do past returns predict future returns?); (2)semistrong form tests (How quickly do security prices reflect public information announcements?); (3)strong form tests(Do any investors have private information that is not fully reflceted in market prices?). From 1970s, foreign studies about it not only are in tensive and broad, but also have produced abundant result s. And research methodologies and models also have made great progress, where event study methodology is an important method. The general applicability of the event study methodology has led to its wide use.In the academic accounting and finance field, event study methodology has been applied to a variety of firm speci ic and economy wide events. Some examples include mergers and acquisitions, earnings announcements and issues of new debt or equity. At present, the positive study on China’s stock market’s efficiency is still in beginning stage, so event study methodology is adopted in this paper to do the positive research of earning announcement. The empirical result of the research shows that it is an important support to China’s stock market’s semi strong form efficiency.