Abstract: The computation efficiency of direct numerical simulation (DNS) for Rayleigh-Bénard (RB) convection in rectangular containers is studied. For unsteady flow of RB convection, the solution of pressure Poisson's equation is a key issue affecting the computation efficiency in the whole solving process. Combined with fast Fourier transform (FFT) and chase method, a direct solution of pressure Poisson's equation is presented. Comparing the precision and speed with hopscotch overrelaxation iteration, it is found that the direct solution of pressure poisson equation for RB convection using FFT is efficient. The results of temperature fields for the plume and the large scale circulation, and the change of Nu number for series Ra numbers are given.