This paper presents a model about the relationship between herd behavior and speculative bubbles. With the help of the computer, the relationship between herd behavior and speculative bubbles is dynamically simulated. The simulation results prove that even if without external disturbance, as long as there exists herd behavior in the speculative market, the market price will irregularly fluctuate. Moreover, the higher the degree of the herd behavioris, the bigger the speculative bubbles are.