Abstract: The paper investigates the output characteristics of a chemical oxygen iodine laser(COIL) affected by nonideal resonator mirrors. Various parameters in long-time thermal deformation of silicon mirrors, such as absorption factor, supporting radius, and thermal load, are considered. The simulation results show that the absorption of mirrors plays a key role in beam quality control. If the decrease in laser output power is required to be less than 20% within the working time of 10 s, the absorption factor must be no more than 3.0×10-4. Furthermore, the beam size affects the supporting radius of four-point supporting. The additional phase aberration induced by mirror supporting deformation can be avoided as the supporting radius is greater than 1.4 times of the beam radius.