Domain knowledge representation is various and domain-concerned. The aircraft assembly process planning (A2P2) is a special domain, a lot of things should be taken into account, and the knowledge representation of A2P2 is complicated. It is focused on the knowledge representation of A2P2 in this paper. Based-on case, the framework of A2P2 knowledge is presented. The main considerations of A2P2 are analyzed, the transformation and reassembly of native A2P2 knowledge is studied, the features of A2P2 is acquired, and the formalizable framework of A2P2 is proposed. With BNF, a formal description of A2P2 knowledge is given.