Based on the nominal reentry trajectory, this paper presents the optimal feedback-gain-coefficients(FGCs) of the reentry longitudinal guidance for the return module1 First of all, the longitudinal planar motion equations are modeled. Secondly, corresponding to the longitudinal guidance law the perturbation differential equations are derived. Finally, according to the Pontryagin minimum principle, the optimal FGCs are given by solving the Riccati equation. Considering the practical feasibility, the varying gain coefficients are approximated to constant ones or gradual constant ones to form the sub-optimal FGCs. And the satisfactory guidance results are achieved.