The energy loss of optical transmission in atmospheric laser communication is analyzed.The main factors affecting the optical power loss of the receiver are discussed emphatically,and a comprehensive simulation is done.The change of the optical power at the receiving end is obtained when the transmitting power is constant and the axial offset of the transmitting antenna and the lateral deviation of the transmitting and receiving antenna system take different values.Based on the analysis of energy loss,the relationship between the offset and the off-axis was discussed under the condition that the minimum optical power requirement is satisfied at the receiver.A method of improving the antenna capture at the receiving end by adjusting the transmitting antenna bias in long-range free-space optical communication is proposed.Under the transmit power of 10000 mW,the distance of 10km,and optical power threshold of 50 mW at the receiving end,the maximum permissible axis deviation between the transmitting and receiving antenna systems is 495.9 mm when the transmitting antenna is not defocused;when the focal shift is 6.69 mm,the maximum deviation is 628.5 mm,which is increased by 26.74% compared with the un-defocused condition,and the area of the lateral offset area between the receiving and receiving antenna system is increased by 60.63%,which effectively reduces the difficulty of long-range receiver-side capture.