A new kind of pneumatic motor with offset swing planetary drive of bevel gears and the application in Capstan is presented; the researched of the calculation of the efficiency of the mechanism is made. Compeered to the former structure, new kind of structure have many advantages over that of the old structure. The pneumatic driving system in the traditional derrick car is: pneumatic motor drives a reducer, and then the reducer in turn drives a capstan. Another pneumatic driving system is: an axial piston motor and an internal cycloid pinwheel drive are applied, so that the reducer and the capstan are combined into one so that simplified the transmission system. In the third version, a pneumatic motor is applied, which can be considered as the combination of pneumatic motor with axial piston and the planetary reducer and, then drives the capstan, so that simplified the transmission system as well. The new kind of pneumatic motor with offset swing planetary drive of bevel gears, which combined the pneumatic motor, the internal planetary reducer with bevel gears, and the capstan together, is the most compact structure. It is of more advantage in the occasion when space is extremely limited. In the paper, two kind of construction of this mechanism is presented, and the analysis of the efficiency of this kind of transmission is made as well.