The invention belongs to the field of lubricating oil and particularly relates to a gas engine lubricating oil composition and a preparation method thereof. The lubricating oil composition comprises 75-95wt% of basic oil, 0-12wt% of a viscosity index improver, 2-12wt% of a cleaning dispersant, 0.1-5wt% of an extreme pressure antiwear additive, 0.1-2wt% of an antioxidant and 0.005-0.03% of an anti-foam agent, wherein the cleaning dispersant comprises polyolefin-succinimide, sulfonate and alkyl phenate; the extreme pressure antiwear additive comprises a sulfur-phosphorus composite type metal organic compound; the antioxidant comprises an organic phenol antioxidant. As the constitution components and the component contents of the lubricating oil composition are optimized, the viscosity maintenance, the oxidation nitration resistance and the alkali value maintenance of the lubricating oil composition are remarkably improved, and thus the lubricating oil replacement cycle of the lubricatingoil composition is prolonged under a high sulfur gas condition.