Abstract: Causes of cracks in SA738 Gr.B lap joints by auto gas metal arc welding(auto-GMAW) and shield metal arc welding(SMAW)was studied in the construction test of steel containment in a nuclear power station. The microstructure and mechanical properties of the coarse grain heat-affected zone were emphatically discussed. The phase array ultrasonic test(PAUT) was firstly used to measure the cracks to confirm the cracks initiation position, and then the optical micrograph(OM) and electron back-scatter diffraction(EBSD) were used to analyzed the microstructure and grain boundary characterize around the cracks. Furthermore, microhardness, room temperature tensile and impact tests were carried out to get the mechanical properties around the cracks in lap joints. The experimental results showed that microstructure and mechanical properties of coarse grain zone in heat-affected zone of lap joints were different in different welding methods, and the mechanical properties mismatch under residual stress caused the cracks of lap joints.