If the known wavelength was inconsistent with the actual wavelength or the wavelength was shifted due to the wavelength instability， the reconstructed image would become fuzzy and the resolution of the image would be reduced. A piecewise correction method for the wavelength instability is proposed that is combined of the self-focusing method based on Tamura coefficient and ptychography， and it can correct the wavelength instability by changing the axial distance at different stages. Based on the Fresnel diffraction theory， we establish the model for the wavelength instability， and it points out that adjusting the axial distance can be used to correct the wavelength instability. The axial distance of each phase can be determined by the self-focusing method based on Tamura coefficient， and the new axial distance is taken into the next stage of the iterative calculation， until the difference between the adjacent two calculations of the axial distance meets the termination conditions. The last axial distance is again taken into the ptychography algorithm to correct the wavelength instability and obtain the distinct reconstructed image， and the correction accuracy is in the nanometer level. Simulation and experiment have proved the feasibility of the piecewise correction method for wavelength instability.