In order to confirm which process is the most important in the blood coagulation cascade, a dynamic model of the function of platelets in blood coagulation is provided based on biochemical experiments. A series of conclusions based on qualitative analysis and mathematical simulation are drawn about the influence of the activation rate of factor VIII and factor IX on the generation of thrombin (IIa). It is evident that the pro-coagulation stimulus must exceed a threshold value to initiate the coagulation cascade. The value is related to the rate of platelet activation, the binding constant d2. The stability of the fixed value is also related to the pro-coagulation stimulus. This article also evaluates the influence of the stimulus strength and the activated rate parameter of platelets on thrombin. The proportion of platelets activated at any given time is designated c. To each c, we obtain a maximum concentration of thrombin. It is evident that when the level of factor IX is below 1% of normal levels, the rate of thrombin generation reduces dramatically resulting in severe bleeding tendency.