Abstract: To collaboratively address the challenges of Yellow Sea algal bloom resource utilization and microalgae cultivation efficiency, this study innovatively employed subcritical water extracts of green tide algae (Ulva sp.) and golden tide alga (Sargassum horneri) as medium additives to investigate their regulatory effects on the growth and metabolism of freshwater Chlorella sp. The results demonstrated that the growth-promoting effect of Ulva sp. extract on Chlorella sp. was significantly superior to both Sargassum horneri extract and the conventional sodium acetate control. At a concentration of 3.6 g/L green tide algae extract, when cultured to the 36th day, the cell density, biomass, chlorophyll a, and carotenoid content were 1.8, 5.7, 5.0, and 3.7 times those of the blank control, respectively, with total sugar and soluble protein yields reaching 7.1 and 6.7 times the control levels. This study confirms the feasibility of utilizing Yellow Sea algal bloom extracts as a low-cost culture medium, providing a novel "waste-to-resource" strategy for algal bloom management and microalgae industrial applications.