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.　©　2022　Chongqing　University.　All　rights　reserved.