The Doppler lidar has been widely used in the detection of atmospheric wind field. An optical fiber Mach-Zehnder interferometer (MZI) used as a new frequency discrimination system for the Doppler lidar, has the advantages of small size, light weight, and high stability. To address the problems of serious energy coupling loss and low coupling efficiency when the commonly used single-mode fiber MZI is directly coupled with the multimode fiber of the telescope receiving system, the multimode fiber MZI is proposed as a frequency discrimination system to improve the coupling efficiency and increase the energy and detection height. In order to solve the problem of interference quality degradation caused by the multimode fiber MZI, the quasi single mode output of the multimode fiber is realized by adjusting the incident angle and disturbing mode. Theoretical calculation results show that the incident angle of the quasi single mode output of the multimode fiber used in the experiment is 0.8° and the diameter of the disturbed mode is 2.2 cm, which are experimentally verified. Through the simulation and comparative analysis of the signal-to-noise ratio of Doppler lidar wind measurement, we find that the multimode fiber MZI used as a frequency discriminator will effectively improve the system detection performance.