Abstract: Based on the practical monitoring data. the paper has made a systematic analysis on lossing amount of gasoline breath for arched roof tank. By comparing those existing problems of six present calculation formulas, new analyzing calculation formulas have been reckoned again and the calculating diagram has been established. It has been proved that the accuracy of new formulas being increased effectively after calculating test. At the same time, it also discusses the saturation of air space in oil tank and achieves valuable experienced data