In this paper, a large vertical cylindrical steel ammonia storage tank was numerical simulated. Strength and stability assessment under earthquake load, wind load, internal pressure and dead weight were performed. It is found the strength of the tank meets the strength requirements according to JB4732-1995 Steel Pressure Vessels-Design by Analysis. The tank is in stability even under the compressive stress at the leeward side of cylindrical vessel under the combined wind load and earthquake load.