Abstract: Natural convection is often practically suppressed or eliminated by lowering the cabin pressure in the ground test of spacecraft. Parameters affecting the optimum pressure are unknown in general, thus resulting in uncertainty of the effect of the suppression. The method determining the optimum pressure via numerical analysis is proposed in this paper. Spacecraft cabins on the ground with various pressures are simulated. Analytical results are compared directly with that of the cabin in orbit, thus figuring out the optimum pressure on the ground to simulate a flying cabin.