Due to the high measurement accuracy and no speed accumulated error, Doppler lidar becomes an important equipment of modern navigation system. In order to improve the performance of three-beam Doppler vehicle lidar, Oren-Nayar model is used to analyze the relationship between the beam pointing angle and the echo power. The dependences of the signal-to-noise ratio and speed sensitivity on beam pointing angle are studied too. The ranges of horizontal deflection angle and the complement of zenith angle are provided. The comparative experiments are conducted with the complement of zenith angle equal to 53° and 24°, separately. Experimental results show that the chosen angles are appropriate.