An air source controlling device for a pneumatic nail-gun includes a body, a choke valve and a switch. The body includes a controlling chamber for connecting a compressed air source, an air passage for communicating the controlling chamber and the pneumatic nail-gun, a switch channel communicating the air passage and an accommodation space, and an air vent hole communicating the accommodation space and an external environment. The choke valve is disposed in the controlling chamber and is switchable between a communicating status and a non-communicating status for controlling the opening or closing between the controlling chamber and the air passage. The switch includes an actuating member slidably disposed in the switch channel and the accommodation space without covering the air vent hole. A sealing member is disposed on the actuating member and is sealed with the switch channel. The actuating member activates the choke valve.