The invention discloses a salt-bath hardening treatment method for obtaining a supersaturated solid solution from a stainless steel surface, belonging to the field of surface treatment of stainless steel. The stainless steel is soaked and activated by using a 5V01.% HF (Hydrogen Fluoride) water solution and subjected to salt-bath hardening treatment for 8-10 hours below the temperature of 480 DEG C; the salt bath contains CNO<->, the mass concentration is controlled to be 50%-55%, and the salt bath also contains K2CO3, N2CO3, CO(NH2), Li2CO3 and other trace elements. As the salt bath adopted by the method disclosed by the invention has low melting point and good liquidity at a low temperature and the CNO<-> in the salt bath has relatively high reduction potential, the stainless steel is subjected to activation treatment and low-temperature hardening treatment below the critical temperature, and active nitrogen atoms have direct contact with the base body, thus a supersaturated solid solution hardened layer can be obtained, the hardened layer has an S-phase structural characteristic, the surface hardness of the hardened layer is three times the hardness of the base body; and the supersaturated solid solution hardened layer has higher corrosion resistance than austenitic stainless steel before the hardening treatment.