Abstract: Q355 steel is an important low-alloy structural steel in the medium and heavy plate field, with a carbon content of 0.12%～0.20%, and it exhibits excellent strength, toughness, plasticity and corrosion resistance. Corner cracking of steel plates is a common problem in medium and heavy plate production, which seriously impairs product quality and service performance, and thus reduces the yield rate of the medium plate mill. Aiming at the corner cracking issue of Q355B steel plates with a thickness of 8～20 mm, this study adopted metallographic observation combined with electron backscatter diffraction(EBSD) technology to characterize grain size and orientation, investigated the microstructural evolution characteristics of corner cracks, analyzed the main causes of corner cracking, proposed improvement measures, and conducted production follow-up verification. The results show that the grain structure of crack-free samples is uniform and the cracks in cracked samples are mainly distributed on the surface and subsurface with a depth of 0.1～0.5 mm, and the grains on the crack surface are coarse with obvious ferrite and recrystallized structures. Based on microstructural analysis, it is determined that the thickness of the intermediate billet has a significant influence on corner cracking. For steel plates with a smaller thickness, adjusting the thickness of the intermediate billet has little effect, and the incidence of corner cracking can be reduced by appropriately increasing the number of finish rolling passes.