Abstract: Rectangular steel pipes made by ultrahigh strength martensite steel cracked after pickling. The cracking mechanism of the steel pipes was analyzed through the simulated pickling test in laboratory, residual stress analysis and hydrogen content analysis. The results show that the cracks are hydrogen-induced cracking. The tensile stress of the inner wall of the corners, original micro-cracks and hydrogen pickup are the main factors leading to cracking. Prevention and improvement measures were proposed from the aspects of steel pipe forming and pickling process.