Lake Ulansuhai is the largest lake in the Yellow River Basin with an area of 293 km². The average air temperature is 7.3℃ and the winter is about six months with a cumulative negative accumulated temperature about 610-800℃·day. The lake water usually starts freezing in the early November and melting in the end of March or in the early April of the following year. The lake is completely sealed by ice in terms of ice-sealing period for about 100-130 days with an ice thickness of 0.63 m in average. To get better understanding in the migration mechanism of ammonia nitrogen at the water sediment interface and the impact of endogenous nutrients on the lake environment during the ice-sealing period, the water and sediment samples were collected at seven sampling sites in early February of 2018 in Lake Ulansuhai. The concentrations of ammonia nitrogen of the water samples were analyzed. The diffusion flux of ammonia nitrogen at the water sediment interface was estimated. The results showed that the concentrations of ammonia nitrogen in the water ranged from 0.55 mg/L to 1.60 mg/L with an average of 1.05 mg/L. The concentrations of ammonia nitrogen in the pore water of 0-5 cm surface sediments were one order of magnitude greater than that in overlying water which ranged from 6.64 mg/L to 18.63 mg/L with an average of 11.92 mg/L. The diffusion flux of ammonia from the sediments to the water was 1.282-4.269 mg/(m²·d). This indicated that a large number of soluble pollutants had been transported to the sediments during the process of freezing. It would be the one of the dominated source of water pollution to the lake environment in the period of ice stability, during the period of ice thawing, and in a period of time after ice thawing.