The invention relates to an assembling and splicing installation method of a large-span tube-truss roof beam. The assembling and splicing installation method comprises the following steps of: dividing a tube-truss roof beam into a roof beam middle span section, a roof beam left span section and a roof beam right span section; dividing the roof beam left span section and the roof beam right span section into a plurality of sub sections, welding respectively, and transporting to an installation field; manufacturing an assembling and splicing jig frame, sequentially arranging each sub section of the roof beam left span sections on the assembling and splicing jig frame, assembling, splicing and welding into the roof beam left span section; manufacturing the roof beam right span section in the same way; and respectively lifting the roof beam left span section and the roof beam right span section in place, welding and fixing, splicing, installing and fixing the roof beam middle span section, respectively abutting the roof beam middle section with the roof beam left span section and the roof beam right span section, leveling by a web member, welding, so as to form an integrative tube-truss roof beam. With the adoption of the assembling and splicing installation method, the geometric dimension of the whole tube-truss construction member is controllable; and due to the assembling and splicing process on the ground, the working efficiency is improved, the cost of large machineries is saved, the safety risk during the lifting process is avoided, in this way, the construction method is especially suitable for the installation of each large-span tube-truss roof beam.