The work aims to compare and study the stress corrosion cracking (SCC) behaviors and mechanism of 304 and 321 austenitic stainless steels in original state, solution treatment, sensitization in simulated catalytic hydrogenation NH4Cl-containing environment. 304 and 321 austenitic stainless steels were prepared to solid solution and sensitization specimens by heat treatment and investigated by the stress corrosion test of U-bend specimens immersed in simulated catalytic hydrogenation NH4Cl-containing solution. Based on corrosion morphology on U-bend and cracking time, SEM microscopic analysis and electrochemical measurements of corrosion or crack morphology were employed to conduct analysis. The cracking time for 304 stainless steel U-bend in original and solid solution states was approximately 25 days and the fracture morphology was transgranular and intergranular fracture respectively. The cracking time for sensitized specimen was 18 days and the fracture morphology was the combination of transgranular and intergranular fracture. 321 stainless steel U-bend did not appear stress corrosion crack after 39 days at original and solid solution states, but the sensitized specimen generated macro cracks after 30 days. From the electrochemical test results, 304 stainless steel at different heat treatment states had obvious pitting sensitivity in NH4Cl and the pitting and stress corrosion resistance of 321 stainless steel in such environment was better than that of 304 stainless steel. 304 stainless steel at different states has higher SCC susceptibility in high temperature NH4Cl environment for different heat treatment, especially for sensitized one. In comparison, the SCC susceptibility of 321 stainless steel in such environment is relatively low. However, sensitization treatment greatly enhances its susceptibility to intergranular stress corrosion cracking (IGSCC), while the solid solution specimen has obvious characteristic of intergranular corrosion.