The invention provides a connecting joint structure of an autoclaved lightweight concrete (ALC) wall board and a steel beam and a construction method thereof. The connecting joint structure comprises the steel beam, the ALC wall board, a first elastic connecting assembly, a first ALC seal board, a second ALC seal board, a double-end bolt and a second elastic connecting assembly, wherein the steel beam is rigidly connected to the lower portion of a floor, and a first vertical elliptical hole is formed in the top of the steel beam; the ALC wall board is connected between the floor of the lower layer structure and the steel beam of the upper layer structure, and the bottom end of the ALC wall board is in rigid connection with the top face of the lower layer floor; the first elastic connecting assembly is rigidly connected under the steel beam and provided with a second vertical elliptical hole; the first ALC seal board and the second ALC seal board are connected between the floor of the upper layer structure and the top end of the ALC wall board and arranged on the inner side and the outer side of the steel beam of the upper structure in a sandwiched mode; the double-end bolt penetrates through the second elliptical hole and the top end of the ALC wall board so as to elastically connect the top end of the ALC wall board with the steel beam of the upper layer structure; and the second elastic connecting assembly penetrates through the first elliptical hole so as to elastically connect the first ALC seal board and the second ALC seal board with the steel beam of the upper layer structure. Thus the technical problem of damage to an enclosure system due to vertical deformation of the steel beam is solved.