In response to the problem of multi-stage tensioning control of long-span cable-stayed bridges under the influence of sag nonlinearity, based on the basic principles of multi-stage equivalent tensioning of cables and sag nonlinearity effects, the multi-stage equivalent tensioning process is theoretically derived, provide correction methods for various influencing factors in theoretical calculations during the construction process, and a new theory for multi-stage equivalent tensioning calculation is proposed. Through numerical analysis and comparison with actual measurement results, the maximum error of tension control using this calculation theory is -1.25%, and the maximum linear error is 3.3 cm. Compared with conventional linear methods, this calculation theory can significantly improve the uniformity and accuracy of cable force calculation, and does not require multiple iterative calculations, meeting the practical engineering application requirements and effectively guiding the cable tension control of long-span cable-stayed bridge construction.