K-TIG welding method was used to carry out welding test on Q345/316 L dissimilar steel,and optical microscope,scanning electron microscope,universal testing machine,impact testing machine,microhardness tester,were used to study the microstructure and mechanical properties of welded joints. The results show that the weld microstructure close to316 L is austenite and skeletal ferrite with columnar crystal distribution,and the weld microstructure close to Q345 is lath martensite. On the 316 L side,a fusion zone about 45 ～50 m wide is formed,and the tiny vermicular ferrite is distributed on the austenite matrix without direction and discontinuously.The fusion zone on the Q345 side produces the Widmanstatten structure,the superheated zone has the formation of sorbite and the phase change recrystallization zone has the fine grain.The tensile fracture occurred at the base material of Q345 side. The joint welds have good impact toughness, and the fracture locations are all in the heat-affected zone of Q345.The fracture surface is ductile and brittle mixed quasi-cleavage fracture with dimples,river pattern and cleavage steps. The microhardness of weld is obviously higher than that of the heat-affected zone and base material on both sides.