The Caohe Aqueduct of the middle route of the South-to-North Water Diversion Project is a type of three troughs with multi-side walls and with three-dimensional prestressing structure. On the basis the long term field test of prestress losses for the rectangle aqueduct concrete structure, a detailed analysis of prestress loss was conducted. The values of the friction resistance coefficients k, μ were obtained. The duct-friction-loss and anchor-loss of various prestressed cable ducts of the aqueduct were analyzed. The losses of prestress due to the transmutation and retractation of the anchorage loop mouth and the losses of prestress due to tendon relaxation were analyzed as well. The stretching effect between the prestress tendon ducts, the duration losses at the construction period with special tensioning procedure and the loading process were analyzed too. The results show that the percentage of transient loss of prestress is not much relation to the stretching stress, that the measured results don ' t agree well with the calculated results to some extent, based on the actual Code for Design ； and that the losses due to anchorage deformation and retractation of tendons account for a large percentage of the total loss. Other factors that influence the prestress losses, such as temperature and subsequent loadings are discussed. In addition, the influence of subsequent loading and prestress losses temperature on prestress loss may can be ignored.