Abstract: Taking a flushing operation of a 25m³ horizontal underground oil tank in an oil depot for example, the article analyzes the safety of the operation based on a method of safety system engineering and pointed out that the reason of safety danger occurred lies in: ① confusions in oil depot's safety management; ② further improvement is required for safety rules of tank flushing; ③ indifferent safety awareness of operators; ④ short of necessary capital input. It is recommended that the safety managing level be raised further; strengthen the establishment of regulations for the oil depot and personnel's safety education and skill training.