An air supply structure of a pneumatic tool includes a housing and a ventilation module. The housing includes an operating portion including an air chamber, a connecting portion including an accommodating space, and a holding portion including an air entering channel and an air exiting channel. The accommodating space is in communication with the air chamber, the air entering channel and the air exiting channel. The ventilation module, disposed in the connecting portion, includes an air inlet in communication with the air entering channel, an air outlet in communication with the air exiting channel, a first air opening in communication with the air chamber, a second air opening in communication with the air chamber, and a ventilation space. Thus, the size of the ventilation space of the ventilation module of individual pneumatic tools is ensured to be uniform to increase the manufacturing yield rate.