Fabry-Pérot etalon is usually employed as discriminating frequency unit in direct detection Doppler wind lidar, and performance of Fabry-Pérot etalon and design of state of incident light beam are important factors in measurement error of the whole systems. The fundamental theory of the Doppler wind lidar based on a triple Fabry-Pérot etalon is introduced. After deep analysis of free spectral range (FSR) and full width at half maximum (FWHM) and peak value interval in two edge channels, the project of parameters design is obtained. The state of incident light beam has an obvious effect on transmission curve of Fabry-Pérot etalon. According to the correlative equation, the effects of incident angle, divergence angle and partial facula permeating through etalon channel designed on transmission curve are provided, and corresponding measurement error is calculated and simulated.