Protecting the ecological sources of urban built-up areas is of great significance for addressing urban ecological problems. With Harbin City, Heilongjiang Province as an example, this study uses the composite performance measurement model based on the ecosystem service theory to determine the ecosystem service according to the urban ecological mixed demand, screen the ecological source of urban built-up areas through ecological performance, and propose protection strategies. The results show that this screening method is suitable for urban built-up areas, suggesting guiding strategies to optimize the protection of ecological sources and strengthen the ecological construction of ecological pedal space. This study proposes a method of using the composite performance model to identify the ecological source of urban built-up areas, which provides a reference for their determination and support for enterprises and governments to manage ecological space and create mixed hybrid landscapes.