Abstract: To ensure the reliability of welding the plug with the specimen of the composite load test to the hi-grade steel special threaded casing，the welding performance of the plug material of the casing is analyzed，and accordingly the proper welding material and welding structure are determined，and in turn the welding process for the said plug is form-ulated. The welding process involves multi-layer and multi-pass welding with two types of welding rods，and pre-heating before welding. Adoption of the post-welding heat treatment process can enhance the weld strength and toughness. The actual operation result shows that the newly-formulated welding process will ensure the connection strength of the plug welding，being up to the requirement for the CAL IV composite load test to the hi-grade steel special threaded casing as specified under Specification API RP 5C5—2017.