The invention provides a method for mounting a high-altitude large-diameter pipe in a complex environment. The method comprises the following steps that 1, a track support is produced; 2, the track support is hoisted and mounted; 3, the step 1 and the step 2 are repeated, and each track support is welded to the other side of a pipe support; 4, sliding trolleys are produced; 5, the sliding trolleysare continuously hoisted to sliding track beams through a winch; 6, the step 1, the step 2, the step 3, the step 4 and the step 5 are repeated, and the corresponding track supports and the corresponding sliding trolleys are mounted on the other pipe support; 7, each first main sliding trolley is connected with an end pipe segment through an electric chain block; 8, one end pipe segment is hoistedto one side of a pipe bracket through the electric chain block on the corresponding first main sliding trolley, and the corresponding first main sliding trolleys and a subsidiary sliding trolley arehorizontally moved through the winch; 9, the step 7 and the step 8 are repeated, and the other end pipe segment is hoisted a pipe bracket of the other pipe support and mounted in place; and 10, a middle pipe segment is lifted through an electric chain block till the middle pipe segment is in butt joint with the end pipe segments to be mounted.