With a heat pump system, the bath sewage water can be recycled to heat the bath water, which effectively saves energy used on the water heating. This study investigates the performance of the water heating system of a sewage-source heat pump used in a public bathroom of a university campus in Shanghai using on-site survey and TRNSYS simulation. It is found that the variation of both sewage temperature and flow rate can influence the system performance. A rise of 1℃ in sewage temperature results in an average increase of 0.03 in the coefficient of performance (COP) of the system and an average decrease of 0.11h in the system heating (namely, operational) time. The increase of sewage flow rate augments the system COP and lowers the system heating time in varied, but gradually decreased, magnitudes. Comparison of the effect of such two factors finds that the system performance is influenced by the sewage temperature more significantly. This study is expected to provide information for the design of water heating system of sewage-source heat pump in public bathrooms.