Abstract: The influencing factors on optical path collimation between transmitter and receiver were analyzed based on long baseline transmittance meter. Aiming at the measurement principle of transmittance meter,the mechanism of optical path collimation influencing transmittance meters measurement precision was presented,which was realized by influencing the receiving intensity of receiver. And the impact of the deviation of transmitting/receiving end optical path collimation on the receiving intensity of receiving end was studied by simulation. The results show that there is a linear relationship between the receiving intensity error and the transmittance meters measurement error,and the transmitting/receiving end collimation has a great influence on the receiving intensity of receiving end, the receiving intensity of receiver can change by 0.02 mW when the transmitter attitude angle changes by 0.01. The change of transmitter attitude angle can decrease the receiving intensity obviously and increase the measurement error, therefore, the probe optical path collimation is an important influencing factor on the visibility measurement.