The on and off wavelengths are obtained by stimulating the Raman tube filled with different gases in AML-2 air-monitoring lidar. According to the characteristic of lidar light transfer path, a new design about light which introduces the idea of beam-splitting based on polarization modulation is presented to improve the real-time performance. Feasibility of the new method is demonstrated and energy loss is evaluated. The simulated results show that the improved system increases the service life of intruments and the real-time performance of lidar. It provide a choice to improve real-time performance of differential absorption air pollution monitoring lidar in practice.