The invention provides a large diameter mirror surface profile in-place detecting method and device. The detecting system is composed of a robot, a laser tracker mainframe, a mirror to be detected, a supporting tooling thereof, a mirror reference positioning device, a target ball and a robot interface device. The robot interface device comprises a magnetic seat, a flexible force sensor and a robot interface. The mirror to be detected is placed on the supporting tooling of the mirror. The target ball can contact the mirror to be detected, and is magnetically absorbed on the magnetic seat. The robot can drive the target ball to carry out dotting and scanning on the machining face of the mirror to be detected according to a pre-planned path to realize the surface profile scanning of the mirror to be detected until the scanning path covers the machining face of the whole mirror to be detected. Through data processing, the surface profile distribution of the machining face of the whole mirror to be detected is acquired. According to the invention, the detection state of the detected mirror is consistent with the machining state, and the method and device are particularly suitable for large diameter mirror milling and on-line surface profile detection of a grinding stage.