The combined influence of the earthquakes and the widespread red-bed soft rock as fill material necessitates the increasing attention on the slope stability in the west region of Yunnan province. To determine the critical height of slope filled by such soft rock under seismic loading, this paper establishes a model simulating a fill slope based on the finite element method and the strength reduction theory. The influence of different slope configurations, including different slope ratios, heights, and side forms on the slope stability is analyzed, and with the stability factor equal or greater than 1.1 as designated in the specifications, the slope critical heights are back calculated. The results show: (1)seismic loading has a significant influence on the slope stability, although its decreasing magnitude slows down with the increase of the slope height.; (2)the fill slope 16m in height is most instable under seismic loading, so the slope at or greater than 16m high is not recommended; (3)whatever the loading forms on the slope, either under the gravity loading only or the combined gravity and seismic loadings, the linear slope is most instable, with increasing stability for concave and step slopes.