In response to the challenges of complex site setup and low measurement efficiency in existing flange pose measurement methods, a high-precision flange pose measurement method based on three-point centering is proposed. A three-point centering model based on the laser ranging is established, and the system parameters of the model are calibrated using the Levenberg-Marquardt (L-M) algorithm. A monocular visual pose measurement model based on an active luminous target is established, and the Sequential Quadratic Programming Perspective-n-Point (SQPnP) algorithm is employed for pose estimation. A portable integrated tool that integrates a laser displacement sensor and an active luminous target is designed, and this tool is mainly divided into a centering module and pose module. The laser displacement sensor is placed and calibrated according to the three-point centering model in the centering module, and landmark points are added to the target topology in the pose module to optimize the matching strategy for the feature points. Measurement errors owing to insufficient manufacturing accuracy are analyzed, and the effectiveness of the proposed method is proven. Experimental results demonstrate that the proposed method exhibits good repeatability and high accuracy. The average three-point centering error is 0.018 mm, maximum flange center distance error is 0.242 mm, and maximum angle error is 0.103°. Moreover, the results indicate that the proposed method can satisfy the requirements of engineering applications.