Atmosphere protection system is a vital functional unit of selective laser melting（SLM）devices,which ensures the low oxygen environment necessary for SLM processing and affects the performance of SLM formed parts directly. The four options with different number and position of the inlets are simulated by FLUENT. The flow field state of the inert gas in the chamber and its influence on the SLM forming process are simulated,and the optimal option is determined. The results show that the shortest inflating time and the best dust removal effect will be obtained when adopting five inlets: one on the right rear side and four on the top of the chamber,and the intake air volume accounts for 1/4,3/4 of the total,respectively.