Abstract: The model of diffuser used on chemical laser's pressure recovery system is built, the fluid field is simulated. The results show that when the supersonic part of the diffuser is 1310 mm long, the background pressure that the chemical laser can lase normally is 7.18 kPa. When the supersonoic length is 1810 mm, the diffuser's recovery pressure rises to 8.25 kPa; when there are 2 vanes in the supersonic flow channel, the diffuser's recovery pressure rises to 8.52 kPa. The lengthening of the diffuser can raise the pressure, but the losing of the total energy will induce the difficulty of the diffuser's start-up. The use of vanes can raise the diffuser's recovery pressure successfully, and the scale of the recovery system could be reduced at the same time.