The Three-River Source Region is an important ecological security barrier in China. Revealing the spatiotemporal evolution characteristics of its landscape types and ecological risks is of great significance for promoting ecological restoration and landscape pattern optimization in the Three-River Source Region. Selecting the Three-River Source Region for a case study and applying the land-use data from four periods (the 1990, 2000, 2010, and 2020), we constructed a landscape ecological risk assessment model for the region based on the landscape pattern index. We then quantitatively assessed the ecological risks and determined the characteristics of their spatial-temporal evolution. The results showed that: (1) The overall landscape ecological risk in the Three- River Source Region tended to decrease from northwest to southeast, and the distribution of landscape ecological risk was closely related to the natural plateau zones and the changes in land cover. (2) From 1990 to 2020, the areas covered by grasslands, water bodies, croplands, and construction land in the Three-River Source Region increased, while the areas of woodlands and unused land decreased. The spatial-temporal changes in the ecological landscape risk were consistent with the characteristics of the changes in the landscape types. The areas categorized as highest, higher, medium, lower and lowest risk areas, while highest and higher risk areas decreased by 9.76%, medium risk areas increased by 1.03%, lower risk areas increased by 8.99%, and lowest risk areas decreased by 0.26%, respectively. (3) Overall, the Three-River Source Region was dominated by very low to medium ecological risk, the areas of which accounted for more than 70% of the entire study area. Overall ecological risks are decreasing, and there is positive spatial autocorrelation of landscape ecological risks in adjacent evaluation units.