Abstract: Reinforced concrete (RC) columns are the most important axial bearing members in the structure.However,factors such as eccentric loads and irregular geometry of members can easily lead to significant torsional effects and thus compression-torsion failure in RC columns.In practical engineering,fiber reinforced polymer (FRP) has been widely used to improve the performance of RC columns.To investigate the mechanical properties of CFRP sheet-strengthened RC columns under compression-torsion failure and the size effect behavior,a three-dimensional mesoscale numerical model is established,the influence of the axial compression ratio,fiber ratio,and structural size on the CFRP sheet-strengthened RC columns are investigated.The results show that the axial compression ratio and fiber ratio significantly affect the torsional properties and size effects of CFRP sheet-strengthened RC columns.As the axial compression ratio increases from 0 to 0.8,the influence of the axial compression ratio changes from a positive to a negative effect,and the nominal torsional strength initially increases and subsequently decreases.Noteworthily,the maximum nominal torsional strength is attained at the axial compression ratio of approximately 0.4.Furthermore,the increase of the axial compression ratio makes the size effect initially weaken and subsequently strengthen.As the fiber ratio increases from 0 to 2.91%,the constraint effect is enhanced.The nominal torsional strength gradually increases and the size effect weakens.Based on the simulation results in this paper,a torsional size effect law (SEL) under compression-torsion loads is established,which can quantitatively describe the influence of the axial compression ratio and the fiber ratio on the size effect of the nominal torsional strength.