The invention discloses a flexible measuring tool for visual detection of a conduit flange end face and a calibration method thereof. The measuring tool consists of a slider for circumferential positioning of a circular hole in the flange, a round boss nut, a clamping claw, a scroll, a rotating shaft, a handle, a bearing, a top cover and a base. The bottom of the measuring tool is provided with acertain number of visual marking points which are arranged on the base and the slider respectively. The flexible measuring tool can adjust expansion and contraction of the clamping claw through rotating the handle, thereby being adapted to flange end faces with different calibers. After a marking point is calibrated, a measuring tool coordinate system under a true scale is established, and the coordinate system can characterize the normal vector of the flange end face, the center point position and the circumferential position of the circular hole in the flange. By identifying and three-dimensionally reconstructing the marking point on the measuring tool end face rather than a pipe end face body, the conduit flange end face detection precision can thus be effectively improved.