The invention provides a lubricating oil composition of a medium-speed cylindrical piston-type engine for a ship and a preparation method of the lubricating oil composition. The lubricating oil composition of the medium-speed cylindrical piston-type engine for the ship is prepared from a benzotriazole derivative, a polymer succinimide ashless dispersant, a calcium naphthenate clearing agent, a salicylate clearing agent, a naphthylamine antioxidant, a barium sulfonate antirust agent and base lubricating oil. The lubricating oil composition has excellent abrasion-resistant performance, high temperature antioxidant performance, clearing performance, water distribution performance and corrosion-resistant performance, and can meet the requirements of high-performance lubricating oil of the medium-speed cylindrical piston-type engine for the ship.