This paper focuses on the effect of the phase offset of Leading-Edge (LE) morphing on the aerodynamic characteristics of a pitching NACA0012 airfoil. Assuming an unstretched camber and using polynomial interpolation, an explicit expression for LE nonlinear morphing is proposed and implemented for the large pitching motion of the airfoil. Flow field results and aerodynamic forces are obtained by solving the unsteady Reynolds-averaged Navier-Stokes equations for both the airfoil’s pitching motion and LE morphing. Furthermore, the index of instantaneous aerodynamic power is used to quantify the work done by the airflow in a dynamic process. According to the instantaneous aerodynamic power and energy map, which denotes the energy transfer between the airfoil’s oscillation and flow field, the airfoil is subject to stall flutter. The results show that LE morphing with an optimal phase offset of 315° reduces the energy extraction from the flow field, suppressing the stall flutter instability. This optimal phase offset is effective at different pitching axis positions of the airfoil. The results signify that LE morphing can suppress stall flutter by advancing the occurrence of the first LE vortex and increasing the nose-down moment during the upstroke period.