The beam divergence angle of lidar returned signals which is directly influenced by propagation properties of Gaussian beam affects the properties of Fabry-Perot etalon (FPE). In this paper, the three-dimensional interference rings of Gaussian beam are investigated and manifested to some trend. Gaussian beam with small divergence may be propitious to obtain higher energy of FPE zero-order or first-order interference ring. For a lidar system, the effects of beam quality factor M2 and expander coefficient on returned signals are discussed. The numerical results indicate that FPE transmission is changed clearly with the height in the near field and leveled off in the far field, and at a given height, it decreases with the enhance of the value of M2 and increases with the expander. It is supposed that laser with lower beam quality factor M2 and the transmitter with higher expander coefficient are chosen and developed to improve FPE transmission and in the near field decrease the measurement error in lidar using an FPE as spectroscope, which is expected to supply numerical foundation for the lidar system.