The invention discloses a construction device used for an ocean steel pipe pile cladding anti-corrosion system and a mounting method. The whole construction device comprises a water mounting device and an underwater mounting device. The water mounting device comprises a fixing ring, an adjustable triangular support, a scaling ladder, a rail and a pedal, wherein the fixing ring is fixed to a steel pipe pile; the adjustable triangular support is mounted on the fixing ring; the pedal is laid on the adjustable triangular support, the rail is mounted on the adjustable triangular support; and the scaling ladder is erected on the pedal. The underwater mounting device comprises a fixing ring, an adjustable triangular support and a diving support, wherein the fixing ring is fixed to the steel pipe pile; the adjustable triangular support is mounted on the fixing ring; and the diving support is fixed to the adjustable triangular support. A cladding anti-corrosion system construction platform can be constructed rapidly and conveniently, and supporting points are provided for underwater work of divers. The device has the positive functions of improving the construction efficiency and effect of the cladding anti-corrosion system and reducing the construction cost.