Abstract: A secondary-throat diffuser and an 8°diffuser are used respectively on COIL. The performance parameters of the two diffusers are measured and compared. The result indicates that the secondary-throat diffuser may effectively insulate the flow disarrangement from the diffuser, avoid the down-stream flow disarrangement into the optical resonator, improve the stability of static-pressure in resonator, and enhance the stability of the output power of COIL. The secondary-throat diffuser is propitious to COIL at high back-pressure.