Abstract: To meet the demand for pipeline project acceptance, the method of detecting underground pipeline diameter in farmland was studied using ground penetrating radar. Based on the energy gradient of pipeline target radar image, the vertex and edge of target pipelines was extracted, and the theory of Least-squares was used to calculate the underground pipeline diameter. The results showed that using ground-penetrating radar on detecting underground pipeline diameter in farmland was feasible, and could detect the diameter of polyvinyl chloride pipeline with the diameter of 75 ~ 110 mm and buried depth of 0.5m . This study provides a new technology for the acceptance of irrigation and water conservancy project.