Leading into the similitude principle of simulation model test of structure, the method is established for calculating the similitude ratio of flexural moment against cracking of the normal section of reinforced concrete flexural members. On the basis of the experimental results, the formula is proposed for calculating the plastic coefficient of sectional resistance moment, in which the height of normal section, the thickness of concrete covering and the percentage of reinforcement are considered. By comparison tests of the flexural moment against cracking of normal section between the large reinforced concrete beams and their simulation model ones, it is proved that the formula can be used for conversion of flexural moment against cracking between prototype and simulation model, and can be used as reference for the design of simulation model test.