A pneumatic roller driving device of a belt conveyor comprises a roller, an inner gear, an outer gear, a pneumatic motor, a bearing and a transmission shaft and is characterized in that the motor serving as a driving device and a speed reducing mechanism are arranged inside the roller, an air inlet and exhaust channels of the pneumatic motor are arranged in a roller shaft, an air inlet pipe is arranged in the roller shaft, the speed reducing mechanism is composed of a pair of mutually-meshed outer gears, an inner gear and an outer gear, wherein the inner gear is meshed with the outer gear, and the inner gear is fixed on the inner wall of the roller. Due to the fact that the belt conveyor driven by the pneumatic roller is adopted, the pneumatic roller driving device has the advantages of being compact in structure, stable in running and small in occupied space, can adapt to underground environments and other humid environments, can prevent the motor from being affected with damp and burned, and an electric shock accident caused by electric leakage of an electrical device to operators is avoided. The pneumatic roller driving device has the advantages of being safe and practical.