Grasslands are important terrestrial ecosystems that have suffered severe desertification. However, the driving factors and mechanisms of grassland desertification remain unclear. Therefore, elucidating the climatic conditions of past grassland desertification, monitoring current desertification dynamics, and identifying key factors are crucial for combating desertification. Here, the probability density distribution of 208 optically stimulated luminescence ages derived from multiple fixed dunes was analyzed to identify the cluster periods of past grassland desertification. We then compared these periods with reconstructed paleoclimatic indicators to infer the climatic conditions associated with these desertification episodes in the Qilian Mountain National Park (QLMNP). Random forest was employed to map spatial patterns and monitor desertification dynamics from 1986 to 2022. Geodetector and residual analysis were utilized to quantify contributions of key factors influencing desertification. Relatively cold and dry climatic conditions enhanced aeolian activity and accelerated the transition of grassland landscape from alpine steppe to desert, triggering grassland desertification in the western QLMNP during 5-5.18 ka. The combination of intensified human activities and a relatively cold-dry climate promoted desertification in the central QLMNP during 1.28-0.74 ka. Current grassland desertification covers similar to 15.2% of the QLMNP, with interactions between climatic and anthropogenic factors accounting for over 50% of the desertification. Less precipitation, high temperatures, and increased population density are the primary drivers of current grassland desertification. This study proposes an integrated framework of historical process-modern dynamics-driver attribution to track the transition of grassland landscape and broaden our understanding of the mechanisms governing grassland desertification.