The invention discloses an installation method of a buried 780MPa pressure steel pipe adjuster, which comprises the following steps: manufacturing of the adjuster, including manufacturing the adjuster in a semi-splayed shape; installation of an upstream steel pipe and a downstream steel pipe, including installing and fixing the upstream steel pipe and the downstream steel pipe; in-place placement of the adjuster, including placing the adjuster in place and controlling the deviations between the adjuster and the centers of the openings of the upstream and downstream steel pipes to be less than or equal to 2 mm; treatment of gaps between the adjuster and the upstream and downstream steel pipes, including controlling the gaps between the adjuster and the upstream and downstream steel pipes to be less than or equal to 2 mm; seam pressing, including controlling the misalignments between the adjuster and the upstream and downstream steel pipes to be less than or equal to 2 mm; and welding, wherein back-step welding is adopted. The installation method has the characteristics of simplicity in installation, good installation quality, short installation time, low installation cost, low manual labor intensity, small residual stress, good stress state and the like.