As the number of elderly people increases and their leg muscles deteriorate, wheelchairs have become an important means of their transportation. In previous research, we proposed a design that combines a double planetary gears’ wheels with the crawler mechanism to enable wheelchairs to climb upstairs and downstairs. However, while climbing downstairs the situation is different from climbing upstairs. The double planetary gears’ wheels cannot be provided with sufficient friction. Thus, the carrier of the planetary gears cannot revolute as if climbing upstairs and the moving on the stairs will be not stable. To solve the problem, we have designed single planetary gears’ wheels to drive the wheelchair. Both theoretical calculations and proportional models have proved that it will generate fewer accelerations than the previous way, which will ensure a safer and smoother process of climbing downstairs.