A measuring instrument for optical axis parallelism was developed. By transfering laser region of 106 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 ℃.