Abstract: A fast and convenient design-method based on UKF parameter estimation is proposed for lunar free-return trajectories. The method proposed avoids calculating the Jacobian matrix and obtains large convergence ability compared with the common differential-correction method. Given that the initial estimate of the free-return trajectory is generated under the two-body Earth-spacecraft model, the difficulty of guessing a good initial estimate is greatly reduced. Through solving the converted parameter estimation problem using the method proposed, the converged final solution can be found under a high-fidelity gravitational model by a few iterations. Despite its simplicity, the method is proves to be quite effective in finding the solution of lunar free-return trajectories with great numerical accuracy.