In view of the actual cable force calibration problem of cable-stayed bridges in operation, the modified equivalent cable length is introduced and the cable force testing method of cable-stayed bridge considering the position of the damper is established on the basis of the cable force principle tested by the original frequency spectrum method. This method can not only meet the convenience of collecting the cable vibration fundamental frequency during the cable force testing, but also consider the deviation effect caused by the arrangement of cable vibration dampers. Therefore, this method has the good theoretical significance and engineering value in use. Finally, through the actual cable force test of Sanshui Bridge, the mentioned method has the good applicability in the projects.