The invention discloses a self-falling adjustable box girder mounting method, and relates to the technical field of road construction. The self-falling adjustable box girder mounting method comprisesthe steps of mounting of a fixing device, mounting of construction guardrails and a guide frame, hoisting of a box girder, precise adjusting of the position of the box girder and the like. The box girder can be mounted and positioned only by precise adjusting in one direction, multi-time multi-directional adjustment is not needed, secondly, because of arrangement of the guide frame, the influenceof the wind speed on hoisting of the box girder is reduced, the construction guardrails are arranged at the two ends of a pier body, and thus station construction of construction personnel is also facilitated.