Abstract: As the length of wind turbine blade increasing, the flexibility grows and the air-elastic stability decreases. Accurately identifying the blade air-elastic stability boundary is of great significance to blade design. However, the wind turbine works in a random environment with uncertainty, which makes the blade air-elastic stability boundary not a definite value. In this paper, the dynamic stall process of FFA-W3-241 airfoil is simulated numerically based on OpenFOAM solver, and the flutter boundary under the coupled pitch and plunge motion is located through the energy method; a sparse grid-based polynomial chaos method is used to quantify the uncertainty impact for the coupled motion. The results show that airfoil flutter boundary is not clear under the slight stall condition, and the probability of flutter onset is 49.48% when considering the uncertainties as the mean and amplitude of pitch angle of attack, pitch reduced frequency, plunge amplitude, and plunge reduced frequency. In addition, the sensitivity of flutter to pitch reduced frequency is the highest, followed by plunge reduced frequency. While the influence of plunge amplitude on flutter is relatively small.