Combined with saturated-unsaturated seepage and consolidation theories, a three-dimensional seepage analysis model of a soil slope in the main stream of the Irrawaddy River was established to investigate the change rule of slope stability under different water level reduction magnitude and reduction speed conditions, and to evaluate the reinforcement effect of the micro wood piles on the slope. The results show that the faster the river water level decreases and the higher the water level is during the flood season, the greater the permeation slope drop near the slope surface is during the water level decrease process, and the lower the safety coefficient is. By installing a group of micro wood piles, the slope stability safety coefficient increased by 4.50%, 9.00%, and 39.60%, respectively, when the height of the wood pile group was 2, 4, and 6 m compared with the case of no reinforcement, indicating that the reinforcement effect of micro wood pile group is good and has some popularization value.