The combined method of Navier-Stokes equation and direction simulation Monte-Carlo method was used to simulate the flow in the reaction chamber, and the relationship between gas flow velocity and plasma density was analyzed by the Langmuir probe detection equipment. Results show that the flow uniformity can significantly impact the plasma uniformity. Under the pressure of 6.5 Pa, the plasma density and flow velocity show a positive correlation, which is consistent with the experiment results in atmosphere pressure. This research verifies that improving the flow uniformity can enhance the plasma oxidation uniformity.