Abstract: To address the insufficient research on the failure behavior and size effect law of reinforced concrete (RC) columns under combined compression and torsion, this paper employs a three-dimensional meso-scale numerical simulation method to establish a numerical model of reinforced concrete columns that can consider the bond-slip effect between steel bars and concrete and the heterogeneity characteristics of concrete materials. The influence of structural size, reinforcement ratio, axial compression ratio, and section shape on the compression torsion failure mechanism of reinforced concrete columns is explored. Results show that with the increase of the axial compression ratio, the diagonal cracks and horizontal angle of the specimen gradually increase, and the failure mode becomes more brittle. As the axial compression ratio increases, the torsional bearing capacity of reinforced concrete columns first increases and then decreases. The nominal torsional strength of reinforced concrete columns under combined compression torsion loads exhibits a significant size effect, and this size effect behavior shows a trend of weakening first and then strengthening with the increase of the axial compression ratio. Increasing the stirrup ratio can significantly improve the torsional strength and weaken the size effect of nominal torsional strength. Subsequently, the numerical test results of this article were compared and analyzed with the calculated values of bearing capacity in various countries, and a size effect law formula was proposed to quantitatively consider the influence of axial compression ratio and reinforcement ratio on the torsional strength of reinforced concrete columns. The calculation method for the bearing capacity of RC columns under compression torsion loads in Chinese regulations has been revised, to ensure the safety of large-sized specimens.