Based on the investigation of the nonlinear dielectric response of 0.95(K0.5Na0.5)NbO3-0.05LiTaO3 (KNN-0.05LT) piezoelectric ceramics, the contribution of irreversible domain wall movement to the dielectric constant with increasing temperature were quantitatively evaluated. Rayleigh law reasonably interpreted these nonlinear dielectric behaviors and successfully separated the contribution of irreversible domain walls movement from the macroscopic dielectric properties. The extrinsic contribution from irreversible domain wall movement, denoted by the Rayleigh coefficient (α), undergoes an abrupt decline during the polymorphic phase transition due to the change of the crystal structure and the domain configuration. The dielectric nonlinearity is significantly reduced in the tetragonal phase zone after the polymorphic phase transition. Furthermore, the tetragonal phase is more stable than the orthorhombic phase.