A new distribution function of energy is introduced and coupled with a velocity distribution function of particle and applied to study a hot fluid field. Thus, a new lattice Boltzmann model is constructed by both energy and velocity distribution functions and Boltzmann equation in the BGK form, which satisfies the conservation criteria of mass, momentum and energy. Thermal convection in a vertical slot is calculated by using the present lattice Boltzmann model. The numerical results have shown that multi-vortexes in the flow field will take place for Prandtl number Pr=1 and Grashof number 1.3×1026. The numerical results are in good agreement with Lee's studies.