The invention relates to a sectional sliding roof steel truss assembling method. The method is characterized by including the following steps of building a construction platform (5), preparing steel truss basic component units (24), installing first piston shoes (20) and second piston shoes (21), installing support trusses (2), installing longitudinal main trusses (22), installing transverse main trusses (23), lifting, assembling and transversely connecting the steel truss basic component units (24), sliding sectional steel trusses (1), putting the sectional steel trusses (1) in position, assembling the other sectional steel trusses (1), sliding the other sectional steel trusses (1), putting the other sectional steel trusses (1) in position, connecting transverse rods among the sectional steel trusses (1), disassembling second piston shoes (21) and sliding rails (3), and disassembling the construction platform (5). The method belongs to the field of steel structure engineering, roof steel trusses are prepared in sections and put in position by sliding, the problems that roof window construction time of an existing operating railway is short and difficulty of the construction is large are effectively solved, the construction efficiency is improved, and when applied in practical engineering, the method can obtain good technical and economic benefits.