The invention discloses a method of detecting an included angle between an optical axis and a mechanical axis of an optical lens. The method includes the steps: placing a target generator on a focal plane of a collimator, connecting a linear guide rail with a high-precision rotary table, placing a three dimensional adjusting rack and an apochromatic microscope on the linear guide rail, placing the above established devices on an optical path of an off-axis collimator, fixing a to-be-tested optical lens to the three dimensional adjusting rack, and tightly combining a flat mirror with the mechanical reference surface of the optical lens. An included angle between an optical axis and a mechanical axis of an optical lens is detected through device adjustment. The problems of determination and correction of the optical axis when the optical lens and a photoelectric device are assembled are solved. Operation convenience is brought for operators.