Backside cracks were found universally in the ultra-low carbon austenitic stainless steel(316LMod) inner linings of urea reactors in recent years, and led to urea reactors leakage. Analyses on the sample revealed that, the backside cracks were caused by the interaction of wet hydrogen sulfide (H2S) and chloride ions (Cl-) stress corrosion cracking (SCC), and H2S played a dominant role. The source of H2S and chloride ions was the leak detection steam. Furthermore, measures to avoid backside cracks in 316LMod inner linings of urea reactors were put forward and verified.