This paper reaches a detection system which use for detecting the three-axis consistency of optical collimator basing on equivalent method. Describing the detect principle and calculating the offset of the image of optical path caused by spectroscope via anglicizing of optical system. Meanwhile, it uses zemax to simulate the offset of optical path. After adjusting the structure of the system, the detection system is calibrated by theodolite whose accuracy is 0.5″. The experiment result shows that the measurement error of the optical axis consistency <0.05mrad.