The torsional structure design of RC members has been widely concerned in recent years. However, the existing design codes are extrapolated based on the test data of small size specimens without considering the impact of size effect, so the safety degree of the predicted values of large size specimens remains to be further discussed. In this paper, the calculation formulas of pure torsional bearing capacity at home and abroad were compared and analyzed. Based on the results of three-dimensional mesoscopic numerical simulation of pure torsional loading of reinforced concrete columns, the size effect law of nominal torsional strength of RC columns was summarized, and the size effect law formula of nominal torsional strength which can quantitatively reflect the effect of stirrup ratio was established. Furthermore, by introducing the size effect coefficient α_h, a suggestion predicted formula for the pure torsional capacity of RC members considering the influence of size effect was proposed, and α_h is divided into three regions, which comprehensively considers the safety and economy of structural design, so as to ensure the safety of the predicted values of large size specimens. By comparing the existing 90 test data, it was proved that the modified formula can effectively improve the safety reserve of the predicted bearing capacity of large size specimens.