Objective Cause for corrosion crack failure of Q235B steel water tank served in a refinery was analyzed. Methods The macroscopic and microscopic behaviors of Q235B steel corrosion crack failure were characterized by means of macro-morphology, microstructure, fracture observation, chemical composition analysis and mechanical property analysis. Results In the humid atmosphere containing H2S formed at the top of the water tank, inter-granular stress corrosion cracks occurred for the Q235B steel. During the corrosion, micro-cracks were induced by permeation of the hydrogen generated by electrochemical reactions into the substrate. Meanwhile, the corrosion products accumulated on the surface of the substrate. Extremely large tensile stress was formed as a result of the combined effect of expansion of corrosion products and occlusion of the coating. Conclusion Combining stress analysis and environment analysis, the corrosion cracking failure mechanism of the Q235B steel sulfur-containing waste water tank was stress corrosion cracking induced by H2S, and the stress was derived from hydrogen permeation and the expansion of the corrosion products.