Abstract: SIMP steel tubes were subjected to transient liquid phase (TLP) diffusion bonding with different isothermal solidification parameters, and then subjected to 1 060 ℃×4 min in-situ normalizing and (780±3)℃×120 min tempering treatment. The microstructure and mechanical properties of the joints before and after heat treatment were studied and compared. The results show that the microstructure of the heat-affected zone of the joint consisted of coarse martensite and retained austenite, and the martensite transformed into fine and uniform tempered martensite after postweld heat treatment. The weld interface changed from a straight line to a curved line after postweld heat treatment, and the interface bonding area increased. After the postweld heat treatment, the tensile strength and microhardness of the joint were slightly reduced, but the tensile strength was still above 700 MPa; the impact absorption energy increased from no more than 9.4 J to more than 40 J; the form of tensile fracture changed from brittle fracture to ductile fracture. The joint after heat treatment had relatively good comprehensive mechanical properties.