The formulas of populations distribution and Doppler cooling force dependent on the velocity of atoms were described by considering the physical model of one dimensional σ + σ - cooling and repumping lights acting on 24 sub level of Sodium atoms. The velocity dependent Doppler force and P(v) were calculated and discussed at the conditions of different detunings of cooling light, different intensities and detunings of repumping lights. The results of calculations based on this model can interpret the experimental phenomena in magnetic optical trap and can be used as reference for selecting the parameters in experiment.