Abstract: Auto\|compatible default theory is a special kind of default theory with many fine features. It is usually a good way to study logic by using clause. On the basis of Reiter’s default theory and Zhang Mingyi’s auto\|compatible default theory, a research into the characters of clausal auto\|compatible default theory was carried out in this paper. First, the theorem of monotonicity with the number of extension is performed. Second, the proof theory of normal default theory is extended to auto\|compatible default theory. Finally several theorems related to default proof, top\|to\|down default proof and belief modification are gained.