Visibility was regarded as one of the most important meteorological complications affecting the safety in the field of aviation and navigation. The alignment status of transmittance meter has played an important influence on the accuracy of visibility measurement in the course of installation and application. A measuring light path alignment method based on scanning mode was proposed and the automatic alignment system was designed based on this method. The system drove the probe beam and received field of view scanning in two vertical directions by electric machinery, then the azimuth angle of the transmitter and the receiver was calculated, and the alignment of the optical path was realized. System test shows that the system could realize the automatic alignment of the optical path of the transmittance meter, the maximum error of azimuth position is 66 ?滋rad, alignment time is less than 10 min. The system has the characteristics of high collimation precision, process automation, easy installation and so on.