Concentrating on trajectory optimization problem of hypersonic reentry under multi-constraints, a differential flatness based numerical approach was studied. By introducing the concept of pseudo input and its additional zero value constraint, flatness of the reentry model was extended. Thus, the original optimization problem was mapped into flat output space, avoiding integral computation and reducing design dimension. The flat output optimization problem was ultimately transformed into a nonlinear programming problem by parameterizing the flat outputs with cubic spline functions. Numerical simulations indicate that the approach presented can generate trajectories that observe multi-constraints rapidly, and have some illumination for engineering application.