Abstract: Small eccentric compression tests on engineered cementitious composite (ECC)-strengthened reinforced concrete (RC) columns and the unstrengthened column as reference were carried out to investigate the influence of the thickness of the ECC reinforcement layer on the compressive performance of strengthened RC columns. The test results show that the ECC reinforcement layer effectively restrains the core concrete, and strengthened columns exhibit obvious ductile failure pattern. Compared with the unstrengthened RC column, the cracks of the reinforced column are thin and dense. When the peak load is reached, the compression zone is not crushed, and the failure process is relatively gentle, with better integrity, exhibiting certain ductility characteristics; The cracking load, peak load and ductility are increased by 107%-236%, 45%-159% and 37.4%-41.3%. The mid-span load-deflection curves are drawn based on the test results, which could be divided into four stages: The elastic stage, the stable crack propagation stage, the maximum load stage and the descending stage. With the increase of reinforcement layer thickness, the strains of ECC and reinforcing bars are smaller under the same load; With the increase of relative eccentricity, the strains of ECC and reinforcing bars are larger under the same load. Based on the theory of concrete structure and mechanical principle, the action mechanism of ECC reinforced layer on the core column was analyzed and the expressions of compressive strength and peak strain of the ECC-strengthened RC columns were proposed. The calculation formula of the bearing capacity of the strengthened column was established and the relative error between the theoretical results and the experimental values is less than 10%, which is in good agreement with the test results. Hence, the established calculation method could provide theoretical support for the application of ECC strengthened concrete column in practical engineering.