Aiming at the passive boundary layer control methods of COIL (chemical oxygen-iodine lase) cavity, three types of experiment rigs were designed. The upper and bottom walls of laser cavity are removable, so the boundary layer control effect with different experiment rigs can be compared. The experiment results indicate that the thickness of boundary layer in laser cavity can be reduced by both slotted wall, mainstream ejected slot and perforated wall. The pressure distribution in cavity, especially in the latter of cavity, is improved. The boundary layer thickness can be reduced further by increasing the sucking rate in a proper range, meanwhile the cavity pressure can be dropped and the COIL power can be increased. But the COIL power also can be dropped with the overlarge sucking rate. Among the three experiment rigs, the mainstream ejected slot is the most sensitive to sucking rate, that when the sucking rate improved to 5%, the COIL power drops obviously. The perforated wall is the most insensitive to sucking rate, and the COIL power has little change while the sucking rate improved from 1% to 7%.