The boost-glide missile is a kind of precision guided weapons, which flies in complex and changeable flight conditions with high requirements for the guidance law. The flight state variation model of the missile was established, the improved optimal guidance law with the impact angle constraint was derived by the optimal control method, and the longitudinal guidance coefficients and lateral guidance coefficients were introduced into the guidance law. The influence of single guidance coefficient on guidance precision was analyzed and the selection method of guidance coefficients was determined. The influence of fixed impact angle constraint and dynamic impact angle constraint on guidance precision were analyzed according to the requirements of different flight missions. The numerical simulation and the hardware-in-the-loop simulation were finished with a small solid propellant boost-glide test vehicle. Simulated results show that the improved optimal guidance law is reasonable with high guidance precision.