Abstract: The land-based total nitrogen pollution carried by rivers entering the sea is an important source of inorganic nitrogen in coastal waters. In order to explore the internal causes of total nitrogen pollution from rivers entering the Laizhou Bay, this paper takes the Mihe River as an example, and analyzes the routine monitoring data from the environmental protection department in 2020-2022 and the encrypted monitoring data from the supplementary investigations in 2023. The total nitrogen pollution load of various industries is calculated, and the sources and seasonal variations of total nitrogen pollution in the Mihe River are systematically analyzed, and suggestions for control and management measures are proposed. The results show that: (1) The total nitrogen in the Mihe River Basin shows obvious seasonal variations, with high concentration in autumn and winter and low concentration in spring and summer. (2) The total nitrogen concentration in main stream of the Mihe River Basin is relatively low in the upper reaches and high in the middle and lower reaches. Among them, tributaries such as the Xier River, Longgang River and Huanglonggou River mainly held higher concentrations of total nitrogen dominated by agricultural land (60.6 mg/L) and residential land (20.4 mg/L), while tributaries such as the Wujingshi River and Nanyang River mainly held lower concentration dominated by mountainous land (6.3 mg/L), barren land (7.6 mg/L) and wetland (6.9 mg/L). (3) The main sources of total nitrogen in the Mihe River Basin are agricultural non-point sources (49.3%) and industrial and urban living sources (43.3%). The factors such as high fertilizer application to agricultural greenhouses, large irrigation water volume, and high loss coefficients of nitrogen and phosphorus are major reasons for the higher proportion of agricultural cultivation sources (45.4%) in the Mihe River. (4) The seasonal variation of total nitrogen in the Mihe River Basin is mainly related to rainfall, agricultural sources, efficiency of wastewater treatment plants, and self-purification capacity of the river ecosystem. The study has shown that the main source of pollution in the high-value area for total nitrogen in the Mihe River Basin is agricultural non-point source pollution, and the focus for control measures is the groundwater and greenhouse cultivation in the Xier River Basin.