This paper proposes a new step size distribution method in variable step size Digital Back Propagation（DBP）algorithm.Several oblique lines are used to estimate the effective nonlinear coefficient which exponentially increases with the distance.Then a step size distribution is determined to ensure the difference of the slope between the oblique lines and the exponential curve is minimum.The optimal step size distribution is used in split-step Fourier method to compensate the nonlinear impairments.Simulation is implemented in a 3 200 km Daul-Polarization Quadrature Phase Shift Keying（DPQPSK）system with single channel rate of 25 Gbaud using the proposed step size distribution method.Simulation results show that the Q-value based on the step size distribution method is increased about 0.5 dB when compared with uniform step size distribution method.The proposed algorithm has larger tolerance on the nonlinear effects with low complexity.