Abstract: In the field of laser fusion, the ablation rate of target material is mainly measured by the diagnostic of thermal wave burn-through images. The planarity of such images could make great influences on the uncertainty of ablation rate. The improvement of the image planarity has been made based on the theoretical and experimental study of the influence of diagnostical spatial resolution. After optimizing the spatial resolution to 29 m on the 420 eV channel of the diagnostic, we proved with SG-II experiment that the planarity area of thermal wave burn-through image has been increased largely and the fluctuation of thermal wave front is only 4.7 ps which is much less than the temporal measurement uncertainty (about 20 ps) of X-ray streak camera.