Pointing to the oxidation failure of the interface of the composite plate, the plates of Q235 carbon steel and 304 stainless steel were prepared and rolled by a two-high mill, and the microstructure evolution of the bonding interface and the austenite grain boundary were studied after the electrolytic polishing in this paper. At the same time, combined with the reaction mechanism of the micro-pore electrolysis process, the mapping relationships between interface element distribution, micro-pore growth, grain boundary energy and gas pressure under different rolling conditions were also deeply explored.The results show that when the reduction rate of double-pass rolling is 30% / 10%, after taking electrolysis there is almost no micro-hole defect at the interface. However, the austenite grain boundaries become a groove with about 2 μm width. When the second pass reduction rate increases to 20% and 25%, the width of austenite grain boundaries decreases to about 1.8 μm and 1.3 μm, respectively, and the energy of grain boundaries decreases accordingly. When the two-pass rolling reduction rate is 35% / 25%, the width of austenite grain boundaries inversely increases to 1.5 μm. The pores on the interface and the austenite grain boundaries also change into the connecting grooves after electrolysis.This is mainly due to the capillary effect of the interface micro-pores, which leads to the rapid electro-chemical oxidation effect on the inner wall of the pores. The reaction rate is positively correlated with the pressure value from the precipitated gas, and negatively correlated with the pore diameter. The more the number of holes, the faster the corrosion, and the wider the groove paralleling to the rolling direction.