The Stress Corrosion Cracking (SCC) behavior of a new ultrahigh strength stainless steel Cr12 was studies by Slow Strain Rate Tensile (SSRT) test and microstructure observation of fracture surface. The results indicate that the neck surrounding area of Cr12 steel tested in air is the crack origin and fast-propagating region, mainly with a quasi-cleavage fracture. In the weak acid solution, the SCC susceptibility of Cr12 steel increases and the fracture surface remains quasi-cleavage fracture, and slippage steps appear in some local areas. In the weak acid spray test, the oxygen is able to fast diffuse through the thin film to the reaction interface, therefore the cathodic reaction is enhanced by the sufficient amount of oxygen and the passive films are destroyed. The Cl－significantly increases the SCC susceptibility of Cr12 steel by reducing the strength and plasticity of the steel. The pitting is the origin of stress corrosion crack and the SCC mechanism of Cr12 steel is able to be explained by using the slip and cracking of passive film theory.