The invention relates to the field of building installation engineering, in particular to a combined wall structure and a construction method thereof. The combined wall structure comprises steel components fixedly connected with a building frame main body, prefabricated wallboards connected with the steel components in a clamped mode, auxiliary keels arranged on the side faces of the prefabricatedwallboards in the horizontal direction, connecting plates arranged on the surfaces of the prefabricated wallboards at intervals in the horizontal direction, main keels are arranged on the side facesof the prefabricated wallboards in the vertical direction, and edge sealing cover plates fixedly connected with the prefabricated wallboards. The construction method mainly comprises the steps of laying the steel components, installing the prefabricated wallboards, fixing the main keels, installing the auxiliary keels, welding the edge sealing cover plates, fixing the connecting plates and the like. According to the combined wall structure and the construction method thereof, the problems that the construction progress is influenced due to inconvenient construction and the structural strengthof the whole assembled wall is low, and the advantages that construction is convenient, so that the construction progress is not influenced, and the structural strength of the whole wall is improved.