To improve the ductility and toughness of 4J36 Invar alloy welded joints, this study investigates the microstructure evolution of laser welded 4J36 Invar alloy joints under different annealing temperatures, and evaluates the changes in the thermal expansion coefficient (TEC) of the joints after heat treatment. The results indicated that the 4J36 Invar alloy joints maintain a single austenitic phase after annealing at different temperatures, but the grain morphology and size change significantly. Compared to the welded joint without heat treatment, the ductility of the heat-treated one increased by 94 %, which reached 39.2 %. The TEC of the Invar joints increases with the increase of annealing temperature. The optimal process parameters are an annealing temperature of 650 °C and a holding time of 1.5 h. Under these conditions, the welded joint exhibits a strength close to that of the base material, with improved ductility and relatively low TEC.