Abstract: In this paper, the experience of calculating natural convection in an cncloscd square cavity is summarized. Numerical boundary conditions of the MAC method are simplified and the accuracy improved. The FTCS scheme of the MAC method can be replaced by leapfrog and Dufort-Frankel scheme to relax the stability limit on the time step. Numerical results for Rayleigh number 10⁶ are given and compared with bench mark results.The influence of different initial temperatures for unsteady flow is discussed. The results of mteady flow are in agreement with theory. Finally, treatment of outflow boundary is discussed.