In recent decades, the warming and humidification of the Tibetan Plateau have aggravated changes in the runoff and sediment transport processes in the headwater area, and the uniqueness of the cryosphere has made them more complex. In this study, abruption tests and IHA-RVA were performed to assess the variation in the runoff and sediment flux of the Tuotuo River before and after the abrupt change from 1986 to 2017. PLS-PM attribution analysis was performed using environmental factors for runoff and sediment flux change attribution. The following three important conclusions were drawn: first, from 1986 to 2017, the runoff and sediment flux of the Tuotuo River changed abruptly around 1998, and the overall degrees of change were 71.2% and 67.5%, respectively; both were highly altered. This indicates that the impact of climate change on runoff and sediment flux in the headwater was not smaller than that of human activity downstream. Second, under the influence of temperature and precipitation, runoff and sediment fluxes from May to October increased significantly, and the degree of abruption was affected by the thawing degree of the soil, river channel, and vegetation coverage. The variation in the sediment flux was dominated by extreme sediment transport events, which were primarily caused by increased rainfall, ice melting, and soil thawing. Third, the runoff and sediment processes in cold regions are complex because of the combined influence of rainfall, glaciers, soil freeze-thaw, and vegetation. Therefore, it is necessary to further study the local region's ecological security and sustainable development downstream.