Based on the feature and optimum purpose of gravity retaining wall, the optimal mathematical model is proposed taking the retaining wall with horizontal bottom as the example. The four parameters: the wall's top width, the wall surface's slope ratio, the wall toe's height and the toe's width are chosen to design parameters. The cost of retaining wall per unit length is chosen to be the objective function; the requirements of the structure, the stability against sliding, the stability against overturning and the bearing capacity of foundation are selected to be constrained functions. Based on the complex optimization algorithm, the optimum design program of gravity retaining wall is developed. The example design showed that the optimum design program can get the best design result with the lowest cost by checking the stability only once. The optimum design is efficient, and the cost is lower than that of the conventional design by comparing their design results. The optimum design program of gravity retaining wall is practicable. Comparing the same height walls, retaining wall reclining on the slope is the most economical.