Off-site reservoirs use water from other watersheds to supplement their water quantity. Water quality is usually better in off-site reservoirs than in onsite reservoirs, because in comparison to onsite reservoirs, watershed areas are smaller and fewer pollutants are collected; moreover, cleaner water is introduced. However, in Taiwan, the water quality of some off-site reservoirs can gradually worsen, and this factor needs to be addressed. In this study, the Liyutan reservoir in central Taiwan was used as an example to demonstrate the process of evaluating pollution in an off-site reservoir. Pollution loads from point sources (PSs) and nonpoint sources (NPSs) were carefully estimated. Domestic sewage and tourist wastewater were considered the major PS loads in this study. The NPS load evaluation was dependent on the results of a verified watershed model, the stormwater management model (SWMM). The observed data in 2015 and 2016 and supplementary total phosphorous (TP) samplings in upstream rivers in 2018 were used to validate the model results. Model calibration and verification were implemented during dry weather and wet weather to ensure the accuracy of the PS and NPS simulations. The results of this study showed that the average total phosphorous (TP) load generated from within the watershed was 9013 kg/y, and that the TP load from outside the watershed was 4545 kg/y. The percentages of TP loads from NPSs and PSs in the watershed were 83% and 17%, respectively. Finally, we used a verified Vollenweider model to convert the TP loads to the TP concentration in the reservoir. The pollution reduction measures and the associated predicted water quality values were assessed using the verified models.