There are two modes of migration in western pastoral areas: spontaneous migration and policy-driven migration. Comparing these two modes can help to better understand the current situation of migration in pastoral areas. Two villages in Nangqian county, Yushu Tibetan Autonomous Prefecture, Qinghai province were selected as the cases for contrastive study. During field investigation, the research was carried out by using semi-structured interview, questionnaire and key informant interview. The results showed that the scale of policy-driven migration was larger and the time was more concentrated, but the pastoralists who moved spontaneously were more successful in finding alternative livelihoods and had lower willingness to return to pasture. Furthermore, this paper analyzes the mechanism behind pastoralists’ decision-making of the two migration modes from the macroscopic, microscopic and mesoscopic perspectives by using the multi-level decision-making framework. The results point out the main influence of urban pulling force and social network on spontaneous migration and the main influence of government subsidy and rural pushing force on policy-driven migration. Accordingly, the corresponding policy suggestions are put forward: transforming the one-time government subsidy into a form that can improve the sustainable livelihood of pastoralists, refusing rigid project arrangement and helping potential immigrants to build social networks.