Abstract: It is important to confirm the technologic parameter（especially the mixed time）of the non-vacuum accumulation leak detection method to study the diffusion state of tracer gas in the accumulation chamber.Based on the CFD simulation and the assumption that a tiny gas mass in the middle of accumulation chamber was used to simulate the leak of tested object，the diffusion laws under the free diffusion and forced convection conditions were both studied.The results indicated that the free diffusion speed of Xenon was slow and the mixed time was long；under the forced convection condition，the diffusion state absolutely depended on the flow field of the fan built in the accumulation chamber and the mixed time was very short.The conclusion here can provide the technical support for the operation of the non-vacuum accumulation leak detection method.