<p>Landslides occur frequently during engineering construction in the West Sichuan Plateau due to the resulting disturbance to the scattered loess with little gravel, but the failure mechanism of such landslides is rarely studied. On 20 November 2020, a loess landslide behind Songpan middle school in the West Sichuan Plateau, China, was formed during excavation for highway construction. It greatly threatens the safety of people and property at the toe of the slope and hinders continuous engineering construction. To investigate the landslide failure mechanism and prevent considerable causalities and economic losses from rapid failure, the landslide stability and evolution after rapid failure are analyzed by adopting FLAC3D software and the depth-integrated particle method, respectively. The results show that excavation under a higher groundwater level condition is a primary reason for the landslide deformation. The displacement ratio indicates that the landslide slides along the interface between the loess layer and the gravel soil layer. Moreover, the increase of shear force due to the localized liquefaction of the loess layer and the decline of the resistance force due to excavation are the failure mechanism of the landslide. Once the occurrence of rapid failure, the landslide will move downwards at a maximum velocity of 4.6&#xa0;m/s and will damage the petrol station, the southwest corner of Songpan middle school, and some facilities in the park. Reasonable prevention and mitigation measures should be taken in these affected areas to avoid casualties. The findings infer that the fluctuation in groundwater level should be given more attention in engineering construction in loess regions with wide hollow regions upstream of the engineering site. This study contributes to understanding the failure mechanism of Songpan loess landslide in theoretical studies and providing reasonable prevention guidelines for the development of mountainous towns in western China in practical engineering.</p>