Based　on　the　fatigue　damage　development　law　and　damage　model　of　concrete,　considering　the　crack　propagation　of　ordinary　steel　bars　and　prestressed　reinforcement　under　fatigue　load,　the　residual　useful　section　area　of　reinforcement　was　used　to　reflect　the　fatigue　damage　of　steel　bars.　Further,　we　considered　the　correction　coefficient　of　the　size　effect　of　concrete　and　steel　bars　and　derived　the　formula　for　calculating　the　section　stiffness　of　prestressed　concrete　beams　of　different　sizes　after　fatigue　damage.　A　multi-stage　variable　amplitude　fatigue　load　test　was　carried　out　on　prestressed　concrete　beams.　The　test　results　were　analyzed,　and　the　accuracy　of　the　equation　was　verified　based　on　experimental　data.　The　results　show　that　the　stiffness　degradation　of　prestressed　concrete　beams　and　the　development　of　concrete　damage　under　the　multi-stage　variable　amplitude　fatigue　loading　test　have　similar　laws.　According　to　the　theoretical　derivation　formula,　the　deviation　between　the　calculated　results　and　the　experimental　results　is　small,　which　has　reference　value　for　the　study　of　the　fatigue　bending　stiffness　degradation　of　prestressed　beams　considering　the　effect　of　size　effect.