It is very important whether a driver can give correct decision and precise operation in emergency． So，it is necessary to judge the emergency degree of environment and provide control algorithm of vehicle motion for ensuring the lives and properties’safety．The emergency degree is firstly given based on relative distance， velocity and drivers’characteristics．Then the control algorithm of vehicle motion based on fuzzy logic is established and is simulated with Simulink．The results show that the higher the emergency degree，the bigger the maximum deceleration is used to control the vehicle．The results also show that the drivers’characteristics have obvious effect on the braking operation．The fuzzy logic is valid to control vehicle’s deceleration．