The invention relates to a method for assembling a large-diameter steel pressure pipe in a long shaft in hydraulic and hydro-power engineering, which comprises the following steps: firstly, a movablegantry crane is arranged above the shaft; secondly, a shaft work platform provided with a circular structure is established; and thirdly, the steel pipe is measured and assembled by utilization of projected points of a laser plumbing instrument. The method has the advantages that: firstly, due to large lifting tonnage of the gantry crane, the problem of vertical transportation of the large-diameter steel pipe in the deep shaft is solved; secondly, due to adoption of the multilayered work platform during the construction process, the method increases the work surface, shortens the assembly time, realizes unprevailing cross-operation under the condition of guaranteeing safe construction, and improves the work efficiency; and thirdly, due to adoption of the high-precision laser plumbing instrument to project points from the bottom up and welding of a pedestal of the instrument on the assembled steel pipe, the method avoids measuring deviation due to repeated assembly of the instrument, solves the problem of measurement and control of the deep shaft, and simultaneously simplifies the construction procedure.