Abstract: Fifty eight CCD frames taken by the 2.4m telescope of Yunnan Observatories on Feb 5, 2013 were corrected for their Geometric Distortions (GDs) with the solved GD pattern by the approach proposed in Reference2. It is found that the GD residuals of the pixel position of stars still exist about ± 0.02 pixels in both X and Y directions while they are bigger at some corners and smaller in the middle part of each CCD frame. A simulation experiment was then carried out to analyze the cause that affects the GD residuals. Our results show that the GD residuals grow bigger when the FWHM (Full Width at Half Maximum) of a CCD stellar image becomes bigger or the GD pattern changes fast. This experiment can explain the variation of the GD residuals of stars after correcting the GDs.