Nitrate eutectic salts is the most widely used heat transfer heat storage medium for solar thermal power generation, but the risk of corrosion under high temperature becomes challenge to the system stability and safety. In this work, the corrosion behavior and mechanism of three austenitic stainless steels 310S, 316L and 321 were studied experimentally in a new quatemary molten salt KNO3-NaNO2-NaNO3-KCl. The composition and morphology of the corrosion products were analyzed by using the methods of XRD and SEM-EDX. The results show that the corrosion rates of three stainless steels are 0.499, 1.460 and 1.895 mu m/year, respectively. Both 316L and 321 have obvious Cr depletion region resulting in intergranular corrosion. By comparison, 310S has the best corrosion resistance in this quaternary molten salt. It is also found that Mo and Ti cannot inhibit the occurrence of intergranular corrosion, but accelerate the dissolution of sigma-phase and TiC in the quaternary molten salt. Besides, the corrosion behavior of 321 based pipe in actual service conditions for a long-time running of about 9 years. Compared with the short-term test, the surface of the metal is thoroughly corroded with the similar corrosion products and the distribution of elements.