Abstract: This study aimed to investigate critical threshold indicators for controlling algal blooms in rivers replenished with reclaimed water, an increasingly common practice for alleviating water scarcity in urbanized regions. With the growing environmental concerns associated with nutrient enrichment from treated effluents, understanding the ecological impacts of water reuse is imperative. The research was conducted using samples collected from a typical wastewater treatment plant in Yiwu City and Chengxi River, which receives effluent discharge. Three dominant algal species from the Chengxi River- Chlorella sp., Microcystis sp., Anabaena sp. were cultured for under different mixing ratios of river water to reclaimed water (1:0, 1:1, 1:2, 2:1, and 0:1). Key parameters including algal growth rate, cell density, and nutrient (nitrogen and phosphorus) uptake were monitored throughout the incubation period. The results revealed that blending reclaimed water with river water significantly elevated the risk of algal proliferation. Chlorella and Microcystis sp. exhibited markedly higher growth trends compared to Anabaena sp., suggesting species-specific responses to nutrient availability and water quality changes. The specific growth rates of both Chlorella sp. and Microcystis sp. reached their maximum at mixing ratios of 1:1 and 2:1, where algal densities in the stationary phase were also the highest. Based on these findings, it is recommended that the mixing ratio of river water to reclaimed water should be maintained below 1:2 to mitigate excessive algal growth and reduce bloom risk. Furthermore, considering algae growth dynamics and nutrient consumption (nitrogen and phosphorus), it is advised that the water allocation cycle—especially during extended periods without rainfall runoff—should be managed such that replenishment does not extend beyond the exponential growth phase of dominant algae. The study provides practical insights and quantitative thresholds for environmental managers to optimize reclaimed water use in rivers while controlling eutrophication and ensuring ecological safety.