A direct detection Doppler lidar system with coherent light path which can work in high altitude area is proposed. Design of the optical path system is improved on the basis of coherent lidar system, the atmospheric molecular scattering light is used for wind speed measurement. The optical path of direct detection system is simplified by photoswitch and monostatic telescope. The photoswitch is composed of polarization splitting prism and 1/4 wave plate. The stability of optical path is improved. The etalon with fixed cavity length is adopted. The wavelength of pulse laser is tunable. The tuning range is about 10 GHz after optimization. The simulation design and equipment selection of the system are completed.