Abstract: The two-dimensional basic equations of fluid mechanics are used to numerically simulate the characteristics of flow between two plates heated from below with horizontal flows, with the Prandtl number Pr equaling 0.72. The numerical simulation results show that for a given reduced Rayleigh number r, with the increase of the Reynolds number Re, there appear steady convective rolls, uniform traveling wave convective rolls, localized traveling wave convective rolls with the entrance region of the horizontal flow, and the horizontal flows occur successively between two plates heated from the bottom. When the Reynolds number Re is between 9.0 and 21.0 and the reduced Rayleigh number r equals 3, the localized traveling wave convection rolls occur in the system and there exists the entrance region of the horizontal flow. The length of the entrance region of the horizontal flow X₀ increases with the increase of the Reynolds number. When the reduced Rayleigh number r is between 4 and 12, and the Reynolds number Re equals 25.0, the localized traveling wave convection rolls occur between two plates and there exists the entrance region of the horizontal flow. The length of the entrance region of the horizontal flow decreases with the increase of the reduced Rayleigh number. And finally, the empirical formulas for the length X₀ of the entrance region varying with the Reynolds number or the reduced Rayleigh number are given in this paper.