On basis of the test results of beams strengthened with CFRP sheets or prestressed CFRP sheets, the paper analyses on the bending capacity of beams strengthened with prestressed CFRP sheets and put forward the formulas for the calculation of the cracking loads and yielding loads.With the concrete's nonlinear property being fully considered in the calculation formulas, the paper suggests that the nonlinear stress graphics should be converted into equivalent rectangular stress graphics by means of equivalent conversion principle. The results calculated in accordance with the formulas suggested in the paper are in good agreement with the test results.