Abstract: The mechanical properties of Q420GJC steel thick plate rolled by a batch of thermo-mechanical rolling were found to be unqualified during acceptance, and the results of several mechanical properties tests were quite different. The causes of unqualified mechanical properties of the steel plate were analyzed by chemical composition analysis, tensile test, hardness test, metallographic examination and other methods. The results show that the microstructure of the steel plate was not uniformly distributed in the thickness direction, and there were great differences in yield strength, tensile strength and hardness. It was suggested to use full-thickness rectangular tensile specimen for test to avoid disputes on the determination of steel plate product performance.