Cropland expansion has become a prominent feature in developing countries and regions. A comprehensive clarification of the causes of cropland expansion is conducive to the adoption of appropriate actions to intervene and adapt to climate change, support rational land use, and balance ecosystem services. Using the Pumqu River Basin (PRB) in the Tibetan Plateau as the study area, we constructed a conceptual model that emphasizes the role of households and explored the causes of cropland expansion based on field survey data. Our results indicate that households’ perception of climate change, families’ non-agricultural income, the numbers of agricultural labor force, and the population pressure faced were the causes of cropland expansion in the PRB. Based on these findings, corresponding policy recommendations to limit the spontaneous cropland expansion behaviors of local households and reduce the ecological loss caused by cropland expansion are proposed.