Objective To study the change of the microstructures and properties of 17-4PH stainless steel after low temperature salt bath nitriding treatment and mid-temperature aging treatment. Methods The thickness and microstructure of the nitriding layer were analyzed using optical microscope (OM) , the phase composition of the nitriding layer was determined by X-ray diffractometer (XRD), the hardness of the nitriding layer was measured by microhardness tester, and the corrosion resistance of the nitriding layer was evaluated by erosion-corrosion experiment. Results The thickness of the nitriding layer increased with the extension of aging time. The change of nitrogen concentration in nitriding layer during the aging treatment reduced Gibbs free energy. This led to decomposition of expanded supersaturated austeinite into Fe4N with face-centred cubic structure. When it went up to a certain concentration, some Fe2N precipitated out. The decomposition rate of expanded austeinite increased with rising temperature. Then, extension of aging time and rise of aging temperature promoted CrN to separate out. The depth of the nitrided layer was increased to 27.4 μm; The active energy of nitrogen-atoms in 17-4PH stainless steel was 216.2 kJ/mol as calculated form Thermodynamics formula; The surface microhardness significantly increased to nearly 1150HV0.1, and then gradually reduced; The erosion-corrosion mass loss rate of 17-4PH stainless steel reached its maximum value of 30.3 mg/(h·dm2) after 50 days of 475 ℃ aging treatment. Conclusion 17-4PH stainless steel after nitriding could reach its maximum surface hardness after aging treatment at 425~475 ℃ for some time. Then, with the increasing aging time, its surface hardness and erosion-corrosion resistant performance reduced.