Abstract: Critical ecological land is one of the most important areas to maintain the stability of ecosystem service function, due to water conservation and resistance to soil erosion for the biodiversity. However, a fierce contradiction is ever increasing between ecological protection and economic development under the rapid urbanization in China. Therefore, a coordinating relationship is greatly significant to maintain regional ecological security and environmental protection. Previous ecological land focuses mainly on the concept, type division, and function zoning. It is also lacking in the identification of critical ecological land at present. Accurate identification of critical ecological land in plateau lakeside areas can contribute to ensuring the stability of regional ecological function, and maintaining the virtuous circle of the ecosystem service function. Taking the Dianchi watershed as the study area, the minimum cumulative resistance model was utilized to identify the critical ecological land using a perspective of ecological land importance. Firstly, the ecosystem service functions were selected to evaluate the importance of ecological lands, such as water, biological, soil, and water conservation, as well as landscape leisure, according to the present situation of land use. Then, the cumulative correction and summation were utilized to superimpose four levels for the comprehensive identification of ecological land importance. Five grades were divided into extremely important, important, generally important, and non-important. Finally, the “Resistance Surface” was obtained to assign land use type, where the ecological land with the extremely important grade was selected as the “Source” of the minimum cumulative resistance model. The critical ecological land was identified in Dianchi watershed using the ArcGIS technology and the minimum cumulative resistance model through COST-DISTANCE tools. The results showed that: 1) The “Source” land was mainly the water area and woodland that were distributed in the north, central and western areas of the basin, covering an area of 984.40 km2 and accounting for 33.71% of the total area. 2) The “Resistance Surface” was divided into the high, middle, and low resistance zones, covering the area of 769.12, 960.06, and 1 190.82 km2, respectively. The high resistance zone was distributed in the middle of the study area, where the main land types were construction land, and roads. The low resistance area included the forest land, water area, and other land types with a high ecological service function. The medium resistance area played a transitional role between the high and low zones. 3) The critical ecological land area of Dianchi watershed was 1 216.30 km2 in 2019, accounting for 41.65% of the total area, which was distributed mainly in the northern, central, western, and eastern boundaries of the basin. The main land types were the water area of 306.39 km2 and woodland of 774.53 km2, which were widely distributed throughout the Dianchi watershed. The identification can contribute to the ecological land to maintain ecological function. Consequently, the minimum cumulative resistance model was also feasible to identify the critical ecological land in the watershed.