In arid open-pit mines, rainfall-triggered slope instability presents significant risks, but quantitative thresholds are poorly defined due to limited integration of transient seepage and stability in low-permeability soils. This study fills this gap by using GeoStudio's SEEP/W and SLOPE/W modules to simulate rainfall effects on a moderately steep-slope (51 degrees average) limestone mine slope in Ningxia's Kazimiao Mining Area (annual precipitation: 181.1 mm). The novelty lies in identifying a 12 h saturation window under intense rainfall (>= 100 mm h(-1)), during which pore water pressure stabilizes as soil reaches saturation, creating an "infiltration buffering effect" driven by arid soil properties (hydraulic conductivity: 2.12 x 10(-4) cm s(-1)). Results show that the factor of safety (FOS) drops sharply within 12 h (e.g., from 1.614 naturally to 1.010 at 200 mm h(-1)) and then stabilizes, with FOS remaining >1.05 (basically stable) under rainfall intensities <= 50 mm h(-1), but drops into the less-stable range (1.00-1.05) at 100-200 mm h(-1), reaching marginal stability (FOS approximate to 0.98-1.02) after 24 h of extreme events, according to GB/T 32864-2016. Slope protection measures increase FOS (e.g., 2.518 naturally). These findings quantify higher instability thresholds in arid compared to humid regions, supporting regional guidelines and informing early-warning systems amid climate-related extremes. This framework enhances sustainable slope management for mines worldwide in arid-semi-arid zones.