A model of imitation and contagion is proposed.There are three parameters:the steady herd coefficient b,the proportion of traders y and the investors' initial average attitude x_1.In the immature market of China,b is relatively biggish.When y is lesser,it can generate herd behavior leaning to one side.It is also found that only under some conditions,such as the drive of leaders,making the proportion of traders reach some degree,the equilibrium of herd behavior can be resulted in through the imitation and contagion within investors.