Abstract: The buffer force of the seat-buffer machine in manned spacecraft is usually designed considering vertical shock only. As a result, the buffer journey may exceed space limit under horizontal shock. To solve this problem, the design process is divided into three steps based on the working principle of the seat-buffer machine. Considering the influences of acceleration, buffer journey and gradient, five-level buffer force is redesigned under utmost horizontal shock landing condition. Results show that redesigned buffer force satisfys buffer journey limit requirement and is more robust to landing conditions.