Abstract: Hydrogen induced peeling in the fusion zone between base metal and austenitic stainless steel overlayer is the main problem of hydro-processing reactors in practical operation. Focused on the widely applied 309L+347L austenitic stainless steel overlaying process,the simulation experiments in the high temperature autoclave indicated the peeling crack germinated at the pure austenitic grain boundaries close to the hardening zone (the microstructure in this region is very complex as a consequence of carbon migration during PWHT(post weld heat treatment) and incomplete mixing of melted base metal and stainless steel fillers) of fusion zone between base metal and austenitic stainless steel overlayers and propagated along the austenitic grain boundaries. Based on the experimental investigation,it is the embrittlement of austenitic grain boundaries induced by the hydrogen accumulation and the high residual shearing stress at the overlay interface that cause the hydrogen induced peeling. The experimental results showed that smaller heat input for 309L overlaying,appropriate PWHT and smaller cooling rate of hydroprocessing reactors are all helpful measurements to prevent hydrogen induced peeling.