The development and utilization of renewable energy are critical for reducing fossil fuel consumption, mitigating environmental pollution, and controlling carbon emissions. Heat pump technology, which applied to low-grade renewable energy, is regarded as a strategic and practical approach to significantly minimize the exergy losses. To meet the economic, environmental, and energy-saving demands, urban wastewater serves as a promising low-grade heat source for heat pumps due to its abundant and temperature-stable properties. However, challenges persist in enhancing heat transfer and preventing fouling. This paper investigates the heat transfer performance of a pillow plate heat exchanger in a wastewater-source heat pump system. Results show that wastewater temperature significantly impacts heat transfer more than flow rate. Compared to traditional immersion exchangers, the pillow plate exchanger exhibits superior performance, higher heat transfer capacity, and COP, and more significant energy savings at higher temperatures. Increased wastewater flow enhances turbulence, further boosting heat transfer, COP, and energy efficiency.