Abstract: The four-point flexure experiments of four steel-concrete composite beams including three beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates and one non-strengthened as reference were investigated. The CFRP plate was transverse stretched by independently designed string system equipment and fixed on bottom of steel beam by rotatable end-anchorage. Experimental results show that the failure modes of beams present typical bending failure characteristics. The ultimate load-bearing capacities of three beams strengthened with prestressed CFRP plates are significantly improved by 31.5%, 28.8% and 47.9%, respectively. Unfortunately, the failure of CFRP plate of 150CFRP(10)-S-C beam due to improper anchorage is premature. The ultimate deflections of 100CFRP(10)-S-C and 150CFRP(15)-S-C are little affected by the reinforcement method, reduced by 0.4% and 1.6%, respectively. The stress distribution of CFRP plate is almost uniform during the whole process, and the material strength utilization ratios of CFRP plates are more than 80% at failure. The prestress losses of CFRP plates after 90 h are not more than 2.5%, and its compensation is convenient. However, it should be noted that this method is highly dependent on the reliability of end anchorage.