Abstract: A paintball sensor installing and positioning structure includes a sensor unit screwed to the bottom end of a paintball dropping conduit. A transmitting end and a receiving end of the sensor unit are received respectively in receiving grooves bilaterally provided on the paintball dropping conduit. Positioning elements protruding from the bottom end of the paintball dropping conduit correspond in position to positioning recesses in a gun body to provide secure positioning. A pin at a lower portion of the sensor unit is inserted in a socket of the gun body. Gas inlet ducts provided in the inner wall of the gun body are separate from a paintball dropping port of the paintball dropping conduit such that gas need not pass through the paintball dropping conduit. Hence, corresponding gas ducts can be connected regardless of the precision with which the paintball dropping conduit is installed in the gun body.