The invention discloses an assembling method of a bridge erecting machine. The assembling method of the bridge erecting machine comprises the following steps that S1, a bridge erecting machine body is assembled on the ground; S2, the bridge erecting machine body is integrally lifted to a first set height, and the structural stability of the bridge erecting machine body is inspected; S3, the bridge erecting machine body is sequentially lifted to a second set height, a third set height and a fourth set height, and then a back supporting leg, a middle supporting leg and a front supporting leg are sequentially installed on the bridge erecting machine body correspondingly; S4, a supporting frame is arranged below the back supporting leg and the middle supporting leg, the bridge erecting machine body is descended, and then the back supporting leg and the middle supporting leg are made to fall onto the supporting frame; S5, the bridge erecting machine is tested. By the adoption of the assembling method of the bridge erecting machine, the construction efficiency is high, the construction quality is easy to guarantee, a site is wide, multi-operation-plane parallel construction can be achieved, and the total duration of a project can be controlled beneficially; and meanwhile, the aerial work amount is reduced to the minimum, and the safety is improved.