Abstract: A method for designing the ballistic Earth-Mars cycler is proposed in this paper, to realize the multiple free returns between the Earth and the Mars. First, half-revolution free-return trajectories subjected to the Earth and Mars orbits are introduced utilizing the solution of the Lambert problem and their gravity assistance. With half-revolution free-return trajectories, an algorithm is developed to search for the ballistic Earth-Mars cyclers. Finally, several ballistic cyclers with high efficiency are listed and assessed. The results can be used for Mars exploration mission designs.