Abstract: The large collimator defocus phenomenon becomes more serious due to the substantial change in temperature and atmospheric pressure during the simulation experiments of space environment. In order to solve this problems that there is an increasing difficulty and a bad accuracy when detecting the defocus of collimator, a new method was proposed to detect the focal plane position and monitor the defocus amount in real time for large diameter long focal length collimator. The major systematic errors which induced by the pentaprism were analyzed, and they were corrected by using the method of updating the reference data. Experiments which have been performed on the collimator of 700 mm aperture and 18 m focal length showed that the detection accuracy of focal plane position can be reached 150 m, and it fully met the requirements of optical test in laboratory that position accuracy of focal plane should be less than 200 m for large collimator.