Abstract: As to obtain the precise optimal trajectory of the surface-to-air missile rapidly, a co-states estimation optimization algorithm was developed based on the Legendre pseudospectral method. The algorithm overcomes the shortcoming of the Legendre pseudospectral method such as there are too many optimization variables in the nonlinear programming (NLP) problem and the slow convergence rate. Additionally, on the premise of avoiding solving the two point boundary value problem (TPBVP), the optimal results satisfying the essential conditions could be computed rapidly. According to the characteristics of the midcourse trajectory optimization problem for a surface-to-air missile, the co-state estimation algorithm and the Legendre pseudospectral method were applied, and the results show that the algorithm takes on fast convergence rate and high optimization precision.