A coupling model of hydrodynamics and water quality has been established based on river boundary conditions. Combined with the characteristic of the ecosystem and the composition of functional water demand in the Yiluo River, taking the water quality target of the water function zone as the co-control factor, and considering the water quality requirements of the Cyprinus Carpio in the breeding season in the Yiluo River Estuary, the self-purification water demand flow was calculated. The results indicate that under the condition of the current status of pollution discharge into river, and meeting the limit of total emission control, the self-purification water demand flow are 14.0m³/s and 8.0m³/s, respectively, to reach the water quality target of class IV; The self-purification water demand flow of maintaining the water quality of normal fish breeding from April to June is 13.62 m³/s, and the self-purification water demand flow of maintaining the water quality of fish in good breeding condition is 35.80m³/s.