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