The invention relates to weather-proof polyester industrial yarn and a preparing method thereof. Weather-proof solid-phase polyester is tackified and irradiated with ultraviolet light to obtain the weather-proof polyester industrial yarn. The strength retention rate of the weather-proof polyester industrial yarn after being aged naturally for 100 days outdoors reaches 90% or above. Neopentyl glycol is of a symmetric structure, after neopentyl glycol is combined into a polyester molecular chain, two huge methyl groups provide shield-shaped protection for an ester bond, and therefore polyester fiber has high hydrolytic stability and weather resistance. Unsaturated double bonds are introduced into polyester, crosslinking of fibers is effectively and safely controlled after the polyester fibers are prepared, the gel content of the crosslinked fibers will be increased greatly, and the mechanical property, the weather resistance, the chemical resistance and the flame retardant property of the polyester fibers are improved substantially.