In this study, the Richards equation is employed for the simulation of saturated-unsaturated seepage field; and then the dam stability is analyzed and assessed based on the derived seepage field and the shear strength formulation of unsaturated soil. Numerical results show that the safety factor of dam appears to increase mildly followed by a rapid decrease during the rapid drawdown process of water level. According to the analysis and visualization of plastic zones and displacement fields, it is found that at the early stage of water level drawdown, the left dam slope shall be more dangerous and collapse firstly before the right dam slope; while the water level drawdown exceeds a certain level, the right dam slope shall be more dangerous due to its rapid decrease. One limitation is that only the lowest envelope of multiple safely factor curves can be achieved if of shear strength finite element method is employed for the stability analysis of slope with multiple slope faces. Core wall plays a water-blocking and seepage-proofing role; and it has a damping effect of water level drawdown.