The invention provides a crossbeam building method of a self-elevating platform cantilever beam. The crossbeam building method comprises the steps that an upper face plate, a lower face plate and subwebs are manufactured; an upper T-shaped profile and a lower T-shaped profile are manufactured, specifically, one of the sub webs is connected with the upper face plate so as to complete manufacturingof the upper T-shaped profile, and the other sub web is connected with the lower face plate so as to complete manufacturing of the lower T-shaped profile; and the upper T-shaped profile and the lowerT-shaped profile are fixed to a jig frame, the two sub webs in the upper T-shaped profile and the lower T-shaped profile are butted in the horizontal direction, and the two butted sub webs constitutea web of a crossbeam, so that building of the crossbeam is completed. According to the crossbeam building method of the self-elevating platform cantilever beam, after the upper T-shaped profile and the lower T-shaped profile are fixed to the jig frame, the sub web in the upper T-shaped profile and the sub web in the lower T-shaped profile are butted under the horizontal state, construction difficulty is reduced, the welding quality is ensured, and butting precision is controlled more easily; and meanwhile, construction safety is improved.