Abstract: As the largest lake in northern China, Hulun Lake has important ecological functions, such as water conservation, biodiversity maintenance and climate regulation, and it plays an important role in maintaining the ecological security barrier in north China. In recent years, with the intensification of the warm and dry climate, Hulun Lake is facing ecological security problems, such as the shrinking of lake area, the disappearance of large reed wetland areas, local grassland degradation, the expansion of land desertification area, and the loss of major populations. The research aimed at water resources, water environment and water ecology. The spatial and temporal distribution pattern and succession of ecological security were evaluated based on remote sensing and GIS technology to call Hulun lake basin from 1990 to 2018. The results showed that the ecological security index of Hulun Lake and its basin was 0.495 and 0.774, respectively, which were in early warning and good state in 2018. Since 1990, the ecological security of the Hulun Lake Basin has experienced three periods of precipitous decline, stable period and recovery period. The ecological security level of Hulun Lake was the lowest, which was mainly distributed in the mouth of Xinkai River, most areas on the west bank of the lake, the center of the lake and the bay mouth area to the southeast of the lake in 2010. The ecological security status of Hulun Lake Basin was the worst in 2015, mainly located in the central part of New Left Banner, the Hailar River Basin and Hulun Valley, especially distributed in the east side of the upper reaches of the Wuerxun River. The main factors affecting the ecological security level of the Hulun Lake Basin were the lake runoff, the proportion of cyanobacteria bloom area and the water resource conservation, while long-term overgrazing, water pollutant concentration and wetland area shrinkage were important factors that limit the further improvement of ecological security. The study revealed that the ecological security of the Hulun Lake Basin is closely related to the water resources, and maintaining the lake surface area of more than 2036 km² can guarantee the higher ecological security level of the basin. In addition, the regional ecological security was affected by the area of cyanobacteria bloom, grazing intensity and wetland area. It is suggested to establish a risk prevention and control system for cyanobacteria blooms, rationally check the livestock capacity, and protect and restore reed wetland to improve the ecological security of local areas.