This paper in the behavioral finanee paradigm mainly studies the t effect and reaction-biases caused by equity change, further introduces the concept of reaction bias degree, introduces the theoretical calculation formula of reaction bias degree, presents a method of CAR calculating reaction-bias degree, analysts evolution of the reaction biases degree. We take the company which Stock Dividends between 1998 and 2004 as sample to investigate stock price change before and after announcement day. Stock Dividends has positive one, besides, the announcement has obvious overreaction. Through calculating reaction bias degree by the method of CAR, we find that reaction bias exists, the degree of which trerds downward.