Abstract: Several methods to detect the optical axes parallelism of multi-sensor electro-optical system were analyzed. The advantages and weakness of each method were compared. A practical method is proposed, which converts laser in infrared band to visible light detectable by CCD with an upconversion plate. A red spot was generated on the plate and retained for a while. The spot generated by the low frequency single pulse laser can be detected easily by a CCD. After some further processing, some parameters of the laser such as centroid and dimension can be measured and the parallelism between laser path and optical observation path can be detected.