The invention discloses a method for identifying the existing prestress of a prestressed bridge based on a complete orthogonalization algorithm, which comprises the following steps: 1) m displacementsensors are arranged on the bottom surface of the bridge from the left end to the right in turn; 2), measuring the vertical displacement {y} N * 1 of the bridge measured by the displacement sensorunder the action of the fixed load P (t),, the amplitude {q} N * 1 of prestressed bridges is calculated from the measured vertical displacement of bridges. 4) Establish the system equation T= ({S} T {S})-1 {S} T {b}; 5)to identify the existing prestress value of the bridge, the existing prestress value T of the bridge is obtained by use a complete orthogonalization algorithm. By establishing the mechanical mapping relationship between the existing prestress of the prestressed bridge and the vertical dynamic displacement response of the bridge, the invention realizes the identification of the existing prestress value of the bridge by the displacement response of the bridge, has the advantages of fast identification speed and high accuracy, and has good engineering application value.