To replace the gas in the slab cassette of slab amplifier more quickly, thus to transport micro aerosol particles produced by xenon lamp irradiation, and prolong the operating life Nd:glass, this paper proposes several different designs of gas flow channel on the baffle of slab cassette and compares their purging effects. Based on computational fluid dynamics (CFD) and dispersed phase model (DPM), the purge flow field in the chamber was obtained, and the purging process of micro contaminant particles was simulated. Through comparative analysis, it is found that the time required to achieve a cleanliness of 100 class is significantly different for different apertures and arrangements of the holes. When the hole diameter is 14 mm, and the holes on the upper and lower baffles are orderly round through-holes, the purging time to achieve 100 class is 205 s, and the gas pressure loss in the cassette is 424.3 Pa. Finally, gas purging experiments of a 4×2 combined slab amplifier show that a purging time of 2-3 min is needed to achieve 100-class cleanliness by the optimized design.