Abstract: Optimizing fertilizer use is one of the main ways to improve soil fertility and reduce the eutrophication of surface water in tea plantations, simultaneously. This approach will play an important role in improving the ecological environment of tea plantations. The study used field data, collected from March to September 2018, and meteorological and hydrological data from 2011 to 2016 in the Mengbang Reservoir Area, Menghai County, Yunnan Province, China, to build a regional model aimed at reducing fertilizer use in a tea plantation based on the soil and water assessment tool (SWAT) model. Four different fertilizer application methods were examined (conventional chemical fertilizer and three combined application methods: 70% chemical fertilizer + organic fertilizer, 70% chemical fertilizer + slow-release fertilizer, and 70% chemical fertilizer + soil modifier). Their effect was calculated using fuzzy comprehensive evaluation to systematically study the optimal application of chemical fertilizers in a Menghai tea plantation. The results showed that: 1) the simulation and observed results of monthly runoff and water quality in the Mengbang Reservoir Area had the same trends (R² = 0.895, Nash-Sutcliffe efficiency = 0.837), and the model fitted well. 2) In terms of overall benefit, the water environment index (0.412) contributed more than the soil fertility index (0.317) and the economic benefit index (0.271). 3) The optimal chemical fertilizer application for tea plantations in Menghai County was the 70% chemical fertilizer + slow-release fertilizer combination. Its soil fertility benefit index, water environment benefit index, economic benefit index, and comprehensive benefit evaluation index scores were 0.96, 0.97, 0.99, and 0.97, respectively. In summary, a combined application of 70% chemical fertilizer + slow-release fertilizer can effectively reduce the risk of eutrophication of surface water in tea plantations and improve the soil fertility, without affecting the economic benefits.