Abstract: Safety is one of the most important factors on oil tank farms. Fire protection cost usually takes 15- 20 percent of that for total tank farm construction. Sometimes million yuan (RMB) is spent on fire protection facilities. With the development of market economy, economic accounting for a tank farm has got to be strengthened. People are paying more and more attention to investment profit. Therefore expert prediction and "Decision Tree" are used for risky analysis on a tank farm fire and fire protection investment. Through calculation, Plan III (Sited in the paper) is considered to be superior, according to which, it is necessary to construct a, fire protection system. But only a small quantity of mobile fire protection equipment is needed. The total economic benefit is the best, which doesn't mean that safety can be ignored, but on the contrary, consiousness of safety should be raised. People are supposed to adhere to the principle of putting prevention into the first place. Suggestions for the development of fire protection system for tank farms are set forth based on combined quality and quantity.