A pneumatic motor with built-in striker mechanism mainly includes a cylinder, rotor set and output shaft lever. The rotor set has a rotary drum and flexible blades. The output shaft lever can be driven by the rotary drum. A chamber is formed into the rotary drum and a striking portion on the output shaft lever is located within the chamber. The built-in striker mechanism is placed in the chamber and has striking bulges protruded on the striking portion, movable hammers sleeved externally on the striking bulge and struts axially located on the rotary drum. This new pneumatic motor has the striker mechanism set into the rotary drum of the rotor set, thus reducing greatly the axial length and volume of the pneumatic motor and realizing ease-of-use.