<p>Selective laser melting (SLM) is one of metal 3D printing technologies, which is popular in mold manufacturing field for its stable working process, high manufacturing accuracy and strong capability of directly forming complex parts. 420 stainless steel is widely used in mold material for its good microstructure and mechanical properties, but 3D printed 420 stainless steel contains martensitic components resulting in poor plasticity and toughness which is the main problem at present. In this paper, the effects of tempering and cooling method on mechanical properties of 420 stainless steel printed were studied. By analyzing the internal laws of material microstructure evolution under different tempering processes, the changing mechanism of mechanical properties such as hardness, elongation, impact strength and tensile strength were revealed. It is revealed that the change mechanism of hardness, strength, plasticity, toughness and other mechanical properties is the carbon precipitation behavior in tempered martensite. The results show that the overall performance is the best when the tempering temperature is 550 °C, and the elongation and impact toughness are increased by 7.17 times and 1.5 times respectively. At the same time, the tensile strength can be increased to 1716.35 MPa with furnace cooling.</p>