Abstract: In order to study the effect of opening on the natural convection and heat transfer, the finite volume method was applied to conduct numerical simulation of a two-dimension laminar steady-state natural convection in open square cavity with two slots on one side. The calculation results show that the Ra and opening have the great effect on temperature field distribution and vortex structure in square cavity. In the same opening, the average Nu and dimensionless flow rate increase with Ra. Under the same Ra, the dimensionless flow rate increases with opening. There should be an optimum opening at Ra >10⁴ when the average Nu is maximum value.