Abstract: In order to simulate the real load-carrying capability of strengthened components in practical engineering, the original concrete components were strengthened and cured with self-compacting concrete while continuously under load. Flexural testing was carried out on 7 RC beams strengthened with self-compacting concrete under secondary load and 2 beams without strengthening. Different initial forces, different strengthening thicknesses and different methods of interface treatment were studied to find the corresponding effects on the flexural capacity and section stiffness of the strengthened beams. Crack distribution patterns, load-deflection curves and development law of steel strain were measured in the test. The results show that strengthening RC beams with self-compacting concrete can significantly improve the flexural capacity, section stiffness, and crack resistance of the RC beams. The flexural capacity of the RC beams strengthened with self-compacting concrete under secondary load was reduced with the increase of the initial load. Based on the test results and the plane section assumption, formulae predicting the lagged strain and the flexural capacity of the beams strengthened by self-compacting concrete were proposed, and the calculation results were in good agreement with the test results.