To research the prestress losses (in the tendon) due to shrinkage and creep of prestressed steel reinforced concrete (PSRC for short), a formula to calculating prestress losses were developed, on the basis of the theory of elastic-creep body, the mean value theorem of the integral calculus, equation of equilibrium, physical equation and deformation compatibility condition. Then the simplified calculation method of the intermediate values is introduced. And the result of the calculation is calibrated with the test data, the formulas are considered to have good accuracy.