Abstract: It is of great significance to study the torsional size effect of concrete structures, which can improve the safety design of torsional bearing capacity of structural members. The heterogeneity of concrete, the bond-slip relationship between steel bars and concrete, and the nonlinear interaction between CFRP sheets and concrete are simultaneously considered, then a three-dimensional meso-scale numerical analysis model is established for the torsional failure analysis of CFRP-strengthened RC beams, and the influence of CFRP sheet configuration rate is analyzed on torsional performance and size effect of CFRP-strengthened RC beams. The analysis results indicate that increasing the CFRP sheet configuration rates can not only effectively increase the nominal torsional strength of RC beams, but also weaken the size effect behavior of the nominal torsional strength of RC beams. Based on the numerical simulation results of this study, a torsional size effect law is proposed, which can quantitatively describe the influence of CFRP sheet configuration rate on the size effect behavior of nominal torsional strength of CFRP-strengthened RC beams, and whose accuracy and rationality are verified upon the existing test data.