The stress corrosion cracking behavior and mechanism of 304L stainless steel in high temperature and high pressure water vapor were studied. Stress strain curve measured by means of slow strain rate tests of 304L stainless steel in various conditions; SEM, 3D Stereo Light microscope and XPS were used to analyze the morphology and element distribution of the fracture area of the sample after oxidation. The tensile strength of 304L stainless steel in normal temperature and pressure water was 730 MPa, and the tensile rate is 94.32%. The tensile strength in high temperature and high pressure water/steam environment was 382 MPa and 379 MPa, respectively, and the tensile rate was 44.98% and 47.38%. The fracture surface of 304L stainless steel after slow stretching was covered with a large number of dimples in three test environments. The positions of the binding energy peaks of 304L stainless steel in the XPS spectra obtained in the high temperature and high pressure water environment and the water vapor environment were almost the same, and the relative intensity of the peaks changed due to different loads. The content of Cr in the surface oxide of 304L stainless steel increased in the high temperature and high pressure water environment, while it decreases slightly in the high temperature and high pressure water vapor environment, after loading. The maximum tensile strength and maximum strain value was familiar in high temperature and high pressure water environment as well as the high temperature and high pressure vapor environment. The diffusion rate of metal elements in 304L stainless steel during high temperature oxidation was affected by additional loading. The composition of the oxidation product on the sample surface changed because of the effect of loading.