Abstract: Objective This study aims to investigate influence of narrow gap laser wire filling welding technology for thick-walled components on microstructure and properties of welded joints, providing a reference for the application of this technology in the main pipelines of nuclear power plants. Methods Narrow gap laser wire filling welding method was used to weld TP316L stainless steel pipes with dimensions of ϕ325 mm×30 mm. After welding, microstructure and grain size and morphology of different regions of welded joints were observed and analyzed. A series of mechanical property tests such as room-temperature tensile, impact, bending and hardness were carried out on welded joints. Results The test results show that weld surface has a good formation, without defects such as lack of fusion, pores and cracks. Transformation from ferrite to austenite in the surface layer is sufficient. Microstructure at the weld center consists of skeletal ferrite and columnar austenite. Microhardness of the weld shows periodic changes. The difference of microhardness values between weld center and heat-affected zone is not very obvious, and the maximum hardness is 211.8 HV. Fracture position of tensile specimen is basically in the base metal away from the weld. The average tensile strength reaches 692 MPa. The presence of a large number of dimples on the fracture surface indicates good plasticity. Impact absorbed energy of welded joints gradually increases from the bottom to the top, showing good toughness. The specimen does not crack when bent by 180°, indicating that the mechanical properties of welded joints are good. Conclusion Narrow gap laser wire filling welding of thick-walled components has a good weld formation, without defects such as lack of fusion, pores, and cracks, and welded joints have good mechanical properties.