1. An analysis method for a regional image, applied to a C-arm device, for analyzing an image datum captured by the C-arm device, the analysis method comprising steps of: providing an indication module, wherein the indication module includes a plurality of steel balls, each said steel ball having a known template coordinate and having a real profile; reading the image datum, which includes plural steel ball image data each corresponding to one said steel ball; selecting a plurality of ROIs (Regions of Interest) in the image datum, wherein centers of the plural ROIs are selected from the template coordinates, and each said ROI includes at least one said steel ball image datum; calculating an average brightness of each said ROI, and adjusting gray scale values in the ROI according to the average brightness; searching each said steel ball image datum, by searching an image profile of the steel ball image datum in the ROI; comparing each said steel ball image datum, by comparing each said real profile with each said image profile and defining a center of the most similar image profile as an image coordinate; and analyzing each said steel ball image datum, by calculating a locational difference between the template coordinate and the image coordinate and determining whether the steel ball image datum corresponds to one said steel ball according to the locational difference.