Cefu Yuangui (冊府元龜) extensively quotes texts from the Book of the Later Han (後漢書), containing numerous variant readings. By employing computer technology to digitize various paper versions of Cefu Yuangui and Book of the Later Han, and through programming comparisons supplemented with manual proofreading, a database of variant readings where Cefu Yuangui cites the Book of the Later Han was established. From this database, 75 sets of ancient and modern characters were sorted out. The majority of these cases involve the use of ancient characters in the Book of the Later Han and modern characters in Cefu Yuangui, with a few instances in reverse, but there are also cases where both texts use a mix of ancient and modern characters. This reveals the characteristics of the times of the emergence and use of ancient and modern characters during the Wei, Jin, Northern and Southern Dynasties, as well as the Two Song Dynasties. Some conclusions drawn from this study can provide references and insights for further improvements of the revised version of Cefu Yuangui.