For a long time,Lack of effective methods to detecting nonmetal pipeline, all kinds of conventional pipeline detection instrument which effect is not ideal.Now,a new method which based on the principle of magnetic of detection nonmetal underground pipeline is being used。In this paper,the author used the detection equipment which based on the principle of magnetic to detect some big diameters underground water pipelines of nonmetal in kunming as an example, Through rational analysis of the data processing,Good detection methods were obtained,and the paper summarizes the magnetic imaging characteristics of the nonmetal underground pipeline in this region and provides guidance method of detection for the similar underground pipeline detection.