Abstract: With the influence of temperature,pressure,medium corrosion and vibration,small defects in the weld can be grown into cracks,which will do harm to the in-service industrial pipelines.Based on the modal symmetry algorithm of defect detection of single weld pipeline,the simulation of modal guided waves in L (0,2) is carried out.According to the symmetry principle,it's easy to extract the defect features.A new Cartesian coordinate is established by using the symmetry surface of the weld defects and weld elbow as the vertical axis respectively.The simulation results show that the asymmetric amplitude is in exponential function with the crack depth,and the method can realize the identification and quantification of defects in double welds and elbows.