The invention relates to a method for mounting a dismountable steel trestle. When a bailey steel trestle is constructed in water, the construction steps are as follows: firstly, inserting and driving first span steel pipe piles by virtue of a floating crane in deep-water area; secondly, mounting a first span bailey beam truss and a bridge deck system; thirdly, lifting a crawler crane onto the constructed bridge deck by the floating crane; and fourthly, constructing the steel trestle in holes by adopting a method for mounting guiding beams and lifting and inserting the steel pipe piles by a guiding frame by virtue of the crawler crane; and when the bailey steel trestle is constructed on the shore, the construction steps are as follows: firstly, levelling the ground, and inserting and driving the first span steel pipe piles by utilizing the crawler crane to suspend a vibration pile driving hammer; secondly, mounting the first bailey beam truss and the bridge deck system; thirdly, lifting the crawler crane onto the bridge, inserting and driving the steel pipe piles for subsequent piers; and fourthly, erecting the subsequent steel trestle by the crawler crane. The invention has the advantages that the requirement on the site is low, mounting is convenient, the dependence of trestle construction to the operating environment is reduced, the influence of shallow water and oversized storm to work ships is avoided, the operation on water is changed into the operation on the shore, the construction progress is sped up, and the requirement on construction machinery is lowered, thus having high promotional value.