In order to investigate the long-term prestress losses of reinforced concrete (RC) beams strengthened with prestressed carbon fiber reinforced polymer (CFRP) sheets, an incremental differential model is presented in this paper by using the conversion elastic modulus method. In this model, the creep and shrinkage of concrete, creep of adhesive layer and interfacial slip at the concrete-to-CFRP interface are all considered. The closed-form solution for long-term prestress loss is obtained by given boundary conditions and load patterns. The effectiveness of proposed model is validated by being compared with experimental results of the relative researchers’ test beams in term of long-term prestress losses. The calculated results show that the proposed model can be used for predicting the long-term prestress losses of reinforced concrete beams strengthened with prestressed CFRP sheets.