A magazine for an air gun is provided with first and second shells; an internal passage; a rotatable shaft including a lengthwise groove on a surface, and a transverse slot on the surface communicating with the groove; a pulley rotatably put on the shaft and having both ends rotatably secured to the first and second shells respectively; a ratchet mechanism mounted on the pulley; a pin inserted into the pulley to dispose in one end of the groove; a line having one end wound on the pulley, the line extending through the passage to have an other end engaged with a projectile support in the passage; a spring put on the line and biased between the pulley and the projectile support; and a spring biased knob disposed on an outer surface of the first shell and secured to the shaft.