The formulas of populations distribution and Doppler cooling force dependent on the velocity of atoms in a non uniform magnetic field 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 influence of velocity dependent Doppler forces on the maximum capture speed and temperature of atoms were discussed at the conditions of different magnetic field, different intensities and detunings of repumping lights.