The invention discloses an optical element surface defect detecting system based on active laser beam scanning. In the detecting system, relative movement is not performed between a to-be-monitored optical element and an optical maser, a guide lens, and a photoelectric probe; point-by-point scanning for the surface of the optical element can be realized only through the two-dimensional rotation of the guide lens; the position of the focal point of a laser beam can be changed in real time during scanning, so that the focal point can always fall on the surface of the optical element. The system can realize on-line monitoring of the optical element in a highlight irradiation environment, a high-vacuum environment and a confined space in a high-power solid laser device. The system disclosed by the invention has simple structure and wide application range, and is flexible to use.