Abstract: A measuring instrument for optical axis parallelism was developed. By transfering laser region of 106 μm to infrared region of 3~5 μm and 8~12 μm with a heat target technique, the parallel error of optical axis between strong laser emitter and infrared sensor of a photoelectric tracking equipment was measured successfully. The structure of measuring instrument was designed as absolutely symmetrical and toy brick assembly shapes to maintain the measuring precision under the abominable conditions. Then, an ingenious electromagnetic switch was used to control and convert target surface for long distance and new assembly techniques and several check steps were taken to improve the measuring precision. Finally various environment tests were carried out to validate the reliability of measuring instrument. Experiment results show that the accuracy of the measuring instrument can reach up to 10″, also can maintain its accuracy ≤10″ under temperature of -30 ℃~+60 ℃.