Experimental research on flexural behavior of the different grade RC beams strengthened with ferrocement mortar through six RC beams is carried out. The matching of original component concrete to the composite mortar strength rank is studied and its influence on bending strength, the crack-resisting capacity and the bending stiffness of RC beams is analyzed. Based on plane cross-section assumption and the experimental results, the formulas of the theoretical ultimate strength capacity and stiffness are brought forward. The calculated results fit well with the experimental results, to provide a theoretical reference for actual engineering designs.