Abstract: In order to reveal the progressive failure mechanism of shallow biased loess tunnels under heavy rainfall scouring and weak rainfall accumulation.The progressive failure characteristics and failure mechanisms of shallow biased loess tunnels under different rainfall modes were systematically studied by numerical simulation. The results show that the surrounding rocks plastic zone starts from the foot of the slope and expands upward along the biased side of the tunnel to the ground surface with rainfall infiltration, which eventually leads to instability. Under the same total rainfall amount, heavy rainfall causes the lining stress to change sharply from stable bias to unilateral concentration in a short period of time, causing the damage to expand rapidly and the structural stability to deteriorate significantly. Continuous weak rainfall leads to the gradual transformation of the surrounding rocks from unsaturated to saturated, which leads to deep softening of the soil and the gradual damage of the lining "local-cumulative-penetrating". Both modes induce instability by reducing matrix suction and the shear strength, but heavy rainfall is more likely to lead to sudden failure, and the threat to tunnel stability is more prominent.