Vehicle collision risk model is proposed for vehicle collision avoidance system in this paper. Probability attributions of collision risk and vehicle kinematical parameters were analyzed at first. Then the risk models based on probability computation for angle collision is established. Simulation evaluation was also implemented at many common scenarios employing micro simulation tool of Vissim. First of all, this paper divided the process of collision for car-following into two stages: leading vehicle decelerates and the following vehicle can not avoid the collision on the condition of it. On the basis of that, the probability of rear-end collision which used to express the risk of car-following was calculated using the method of total probability. What's more, a deceleration probability density function model was developed. In the analysis of the condition on which collision occur while leading vehicle decelerating, brake system response time and sticking coefficient were taken into account and this make the model forceful to reflect the real traffic situation. To evaluate performance of the new method, value of risk was calculated as a function of the data generated from Vissim. Simulation results indicate that the new approach is easer to reflex safety situation and more suitable for intersection collision avoidance system.