To improve the impact angle and strike accuracy for anti-tank missiles of helicopter,The terminal impact angle constraint for missile with relative motion relationship between the missile and the target is given based on the relative motion and collision triangle relationship between the missile and the target.The relational expression of the variable bias proportional guidance law based on the impact angle constraint is derived.Through comparison with ballistic shaping guidance law and segmented variable gain proportional guidance law,this guidance law has high guidance accuracy with less overload change and control energy consumption.The research on this guidance law shows that it can meet different simulation conditions and has the higher impact angle accuracy.The research of this guidance law shows that it can meet different simulation conditions and has the higher impact angle accuracy.With the increase of the impact angle constraint,the changes of missile overload and control energy consumption intensify.The influence of the guidance law on the trajectory is further discussed.The analysis of bias term and the angular velocity changes of line-of-sight angle between missile and target shows that the guidance command is initially dominated by the bias term,when the trajectory climbs,and subsequently transforms into proportional term,which completes the impact angle constraint and also ensures the convergence of the trajectory.