Abstract: In order to solve the technical defects of the conventional inner detector and to meet the actual demands of China-Russia Eastern Gas Pipeline Project, a new type of high density polyurethane inner diameter detector for large diameter pipelines before service was developed. The intelligent diameter measuring device mounted on the main body of high density polyurethane foam can not only find the change of the inner diameter of the pipeline and record the deformation position, but also accurately identify the number of girth welds of the pipeline, so as to verify the girth weld information in combination with the pipeline construction records. The working principle, structural composition and design and implementation of the electronic system of the detector were discussed. The detector has been applied to the site inspection of China-Russia Eastern Gas Pipeline Project. The results show that, compared with the conventional cup-shaped steel skeleton intelligent diameter measuring detector, the required operating thrust of the detector is smaller, the allowable operating speed is higher, the structure is more robust, and the maintenance is simple. While ensuring the detection accuracy and detection rate, the detector brings dramatic decline in use cost and important engineering significance for the geometric deformation detection of large diameter pipelines.