Based on informational asymmetry,this article builds up a false-signaling game model in security market.The game model illuminates the mechanism of Herd Behavior in the stock market by a way in which the more-informed investor cheats the less-informed investor through the derivative financial market .Meanwhile,it makes a conclusion of separating equilibrium for common investors'security investment.Finally,the article gives some policies for reference for both the development of Chinese security market and the opening as well as development of Chinese derivative financial market.