Under the horizontal earthquake excitations，reinforced concrete（RC）columns such as the piers of curved girder bridge or the side columns and corner columns of the irregular reinforced concrete frame structure may be subjected to a combination of shear force and torsional moments and often produce brittle failure，due to the eccentric center of gravity of the superstructure. In addition，brittle failure often aggravates the size effect of reinforced concrete structures. In order to study the seismic performance and size effect of RC columns under combined shear and torsional loadings，a three-dimensional meso-scale numerical model of RC columns under combined shear and torsional loadings with different torsion-to-moment（T/M）ratios is established. Then，the effect of torsion-to-moment ratio on the seismic performance and size effect of RC columns subjected to shear-torsion combined loading is studied. The results show that the RC column is more brittle and the bearing capacity is lower under the combined action of shear and torsion. The increase of torsion-moment ratios leads to the decrease of deformation capacity，ductility capacity and energy dissipation capacity. The size effect on shear strength of RC columns is enhanced within a certain range due to the presence of torque. The related equations of shear and torsional capacity proposed in Chinese regulation are compared and modified to ensure the safety of the predicted results.