Eight specimens, including two strengthened after loaded to yield level to imitate strengthening with some damage and one strengthened under a sustained axial load to imitate strengthening under service condition, were tested under constant axial load and lateral cyclic load to investigated seismic performance of RC columns strengthened with carbon fiber reinforced polymer sheet (CFRP sheet). The ductility increment with the confinement of CFRP sheet was studied by the strain development and distribution of CFRP sheet. Based on the experimental results, a confinement factor of CFRP and an equivalent transversal reinforcement index were suggested. Thus, the seismic design method of current Chinese seismic design code for RC column can be directly used in determining seismic strengthening amount of CFRP sheet.