Recently, application of concrete filled steel tubes (CFST) columns has been widely trended due to their several advantages. Some of the CFST columns may been damaged due to different problems and need to be strengthened. One of the methods which can be applied to improve the efficiency of the damaged columns is the strengthening using fiber reinforced polymers (FRP). The present research investigates the effects of strengthening deficient CFST columns trough numerical and experimental studies. The numerical simulation has been adopted using ABAQUS software in 3D simulation method, and the experimental test has been done using steady pressure test machine. The defects created horizontally and vertically at the center of the columns. The specimens include one control and four deficient specimens which two of them were strengthened using four carbon fiber reinforced polymers (CFRP) layers. The results show that the horizontal deficiencies in CFST columns resulted the most reduction in load bearing capacity in comparison with Control (-46%). Strengthening using four CFRP layers (two-transverse and two-longitudinal) resulted in improving load bearing capacity appropriately (approximately %27 increment in comparison by the deficient column). The application of CFRP improved confinement strength and controled the local failures.