Abstract: Cold bending deformation processing experiments were conducted on hot-rolled Q355B steel plate with a thickness of 20, 42 mm. The optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and ASPEX scanning electron microscopy the macro-micromorphology were used to characterize and analyze the microstructure and non-metallic inclusion characteristics of Q355B hot-rolled plate. The ferrite(F)-pearlitic (P) banded microstructure was rated and defined according to GB/T 34474—2017 Evaluation of Strip Structure in Steel B series standard. The effects of plate thickness and microstructure on the cracking of Q355B hot-rolled plate cold forming deformation were studied. The results show that during the cold bending deformation process, the cracking on the lower surface of the hot-rolled plate with a thickness of 20 mm is mainly caused by the F and P banded structures distributed along the rolling direction, which are greater than 3.5 grade.The hot-rolled plate with a thickness of 42 mm almost completely cracks along the rolling thickness direction, this is because the microstructure near the surface fracture of the steel plate is mainly composed of bainite (B) and martensite and austenite (M–A), and the bottom surface undergoes significant tensile stress during the cold bending, resulting in stress concentration under the combined action of strong tensile stress and surface hard structure. The microstructure of Q355B steel plate with different thickness directions is mainly composed of hard phases such as F and P with banded grades greater than 3.5, B and M–A, and the microstructure characteristics are the internal cause of cracking during the cold bending deformation process of the steel plate, and the large non-metallic inclusions are the inducing factors for cracking.