A new lattice Boltzmann equation(LBE) model is developed based on the previous LBE model and the conservative criteria of mass, momentum and energy. The result shows that the LBE model is further improved in an external force field. The relation between the buoyancy strength parameter and the Grashof number is obtained through the recovery of dynamic equations. The viscosity transport term is obviously improved by comparing the derived momentum equation with Navier-Stokes equation. It is shown that the viscosity stress not only depends on the velocity gradient and the compressibility of the fluid, but also depends on the gradient of unsteady internal energy and unsteady momentum flux. The flow field with nonuniform temperature has been simulated by using the model. It is shown that the model is valid both in theory and in numerical experiment.