The invention discloses a tunnel support device of a coal mine goaf. The tunnel support device comprises more than two steel pipes, wherein the steel pipes are arranged along the surrounding contour of a tunnel and are vertical to the plane of the surrounding contour of the tunnel; more than two steel brackets are arranged in an area defined by the steel pipes in a surrounding manner and are vertical to the steel pipes; more than two anchor bolt holes are formed in each steel bracket, an anchor bolt is arranged in each anchor bolt hole, a flexible formwork and a reinforcement net are sequentially arranged on each anchor bolt in a penetrating manner, concrete is arranged in each flexible formwork, and the two adjacent flexible formworks on the same steel bracket are mutually connected. The tunnel support device has the advantages that the strength of the support structure is high, and the stability and reliability are realized. A construction method of the tunnel support device of the mine coal goaf has the advantages that the difficulty in support operation is low, the support effect is good, the construction speed is high, the removal of the formworks is not needed, and the suitability is high.