The oil and gas pipeline sizing technology is an important tool for detecting changes in the pipe diameter and ensuring pipeline safety. To address the problems that the traditional thin aluminum discs cannot detect multiple defects and cannot be reused, this article designs a flexible sizing sensor, which measures the strain change when the resistance is encountered through the resistive strain gauges placed in the flexible polyurethane sizing discs to achieve quantitative detection of defects in the pipe. The coupling model between the strain gauges and the polyurethane sealing disk is formulated with ABAQUS software, and the strain output characteristics are analyzed when tensile and compressive deformations are generated. The function of the flexible sizing sensor is tested by a flexible sizing sensor performance testing experiment bench. Finally, the BP neural network and the genetic algorithm are combined to derive the structural parameters that make the sensor detection performance optimal. For the DN200 pipeline, research results show that when the outside diameter of the sizing disc is 97. 5% of the inner diameter of the pipe, the thickness of the sizing disc is 19 mm, and the sizing disc flap spacing is 8 mm. The flexible sizing sensor can better characterize the size of the depression, which has certain engineering application value for oil and gas pipeline deformation detection operations.