The invention discloses an auxiliary hoisting platform for an underground diaphragm wall reinforcement cage and a construction method for the auxiliary hoisting platform for the underground diaphragm wall reinforcement cage. The auxiliary hoisting platform comprises a reinforcement cage machining platform and a supporting truss. The top of the reinforcement cage machining platform is provided with a lifting lug, the bottom of the reinforcement cage machining platform is provided with supporting points supporting baffles and arc pieces, and the outer surface of the reinforcement cage machining platform is provided with limit pins used for fixing the reinforcement cage. The supporting truss is connected to the top of the reinforcement cage machining platform through a rolling shaft in a pivot joint mode. Tie pieces are connected between the reinforcement cage machining platform and the bottom of the supporting truss. The construction method comprises the steps of manufacturing the reinforcement cage on the reinforcement cage machining platform, lifting the auxiliary hoisting platform, opening the supporting truss and lifting the vertical reinforcement cage into a groove section. The auxiliary hoisting platform for the underground diaphragm wall reinforcement cage and the construction method for the auxiliary hoisting platform for the underground diaphragm wall reinforcement cage have the advantages that the weight is light, the rigidity is large, materials are saved, and the construction quality of the reinforcement cage is high.