Glacial debris flows, which are induced by ice and rock avalanches, pose an increasing threat to human life and critical infrastructure under climate change. Therefore, the movement characteristics of debris flows induced by ice and rock avalanches have attracted increasing attention, but the understanding of the effects of ice and rock avalanches on the mechanism of debris flow mobility remains incomplete. To study the mechanisms of the enhanced mobility of debris flows and simulate the impacts of ice and rock avalanches, we employed a servo-hydraulic-controlled ring-shear apparatus to examine the undrained shear behaviour of moraine material under monotonic and impact loading conditions. It is found that impact loading caused a significant reduction in the shear strength of moraine, with the mobilized friction angle decreasing by 16.1% (from 20.6 to 17.3 degrees) and the residual angle of internal friction decreasing by 52.9% (from 7.0 to 3.3 degrees). After moraine failure (at the peak shear strength), the moraine material exhibited rapid shear strength reduction behaviour under impact loading conditions owing to the lower strength weakening coefficient and smaller slip-weakening distance. Compared with the monotonic loading ring-shear test results, the liquefaction potential was greater, and the shear energy needed to reach the same pore water pressure ratio and ultimate pore water pressure was much lower under impact loading. This suggests that under impact loading, moraine is more susceptible to liquefaction and exhibits mobility characteristics. Our results provide new insights and increase the understanding of the mechanism underlying the enhanced mobility of glacial debris flows induced by ice and rock avalanches.