Abstract: The trajectory optimization for low thrust electric propulsion has been researched in the paper.The orders of the acceleration are of 10 -2 ～10 -4 .The indirect approach is used in optimization.The adjoint equations are integrated backword to avoid the initial guess of adjoint variables by means of the transversality condition.The earth escape trajectory and low orbit transfer optimization are numerically resolved by iteration method.The results show the optimal transfer orbits are almost circular,and transfer time is long.The magnitude of low thrust is inverse proportion.