The high temperature produced in fire has a great influence on the pretensioned pre-stressed concrete beams. Due to the decrease of the effective prestressing force in the structure, the compressive stress spread over edge girth in tension is reduced, and structural cracks appear early, resulting in the decrease of service performance and durability. To detect the effective prestressing force of the pre-stressed concrete beams exposed to fire quickly, thirty-two pieces of beams exposed to fire were taken as samples. The regression formula of spalling depth ratio and prestress loss ratio was obtained by measuring the permanent stress of steel strand of concrete girder bridge after exposed to fire, and the applicability of the formula was verified by an experimental and finite element analysis on ultimate bearing capacity. The results show that the phenomenon of prestress loss of beam prestressed steel bundle exists. When the spalling depth of concrete exceeds 1/3 of the net protective layer of steel strand, the prestress loss cannot be ignored. The thickness of the steel strand protective layer should be increased appropriately for pretensioned prestressed concrete structures in fire resistance design. When the spalling depth of concrete exceeds 2/3 of the net protective layer of steel strand, the rate of prestressing loss due to high-temperature creep exceeds 10%, resulting in severe decrease of the girder stiffness.