To address the issues of poor accuracy and reliability in existing underground pipe diameter measurement methods, this paper proposes a new approach that combines laser ranging with geometric relationships. A laser device is positioned inside the pipeline, and by strategically placing the laser, it emits beams in three directions towards the inner wall of the pipe. The distances are measured, and a formula is derived to calculate the pipe diameter based on the laser measurements. This method enables accurate measurement of urban underground pipelines, and a technique for reducing measurement errors through alignment is introduced. Partition experiments were conducted to verify the correctness and feasibility of the theoretical derivation. Prototypes were developed, and experiments were performed using actual pipelines in a simulated laboratory environment. Results show that, in an ideal laboratory setting, the absolute errors for pipe diameters of 400, 600, and 800 mm are within 10, 9, and 6 mm, respectively, with relative errors of 2. 5% , 1. 5% , and 0. 75% . The measurement accuracy remained high even in partition experiments. Although the error in real-world environments is slightly higher than in the lab, it still meets practical measurement requirements. The system enables fast and accurate measurement of underground pipe diameters with high reliability and a simple measurement structure, making it suitable for field applications.