The invention relates to a large-span overpass widening project narrow space beam slab mounting method. The large-span overpass widening project narrow space beam slab mounting method comprises four steps such as field environment data collection, checking computer equipment model selection, beam slab transportation and beam slab transportation and hoisting. According to the large-span overpass widening project narrow space beam slab mounting method, the floor space and use area of the construction operation surface and assistant facilities during bridge widening operation can be effectively reduced so as to effectively reduce the bridge widening construction cost and damage and influence on the surrounding environment and effectively enhance the construction efficiency and effectively improve the construction quality; and therefore, multiple demands on the bridge widening construction operation under a complicated construction condition are effectively satisfied.