In order to resolve the problem of energy-saving overestimation to variable frequency, the experiment of flowrate and power of an mine main fan, which was adjusted by variable-frequency, adopted by the coefficient experiment system of a mine main fan performance developed by ourselves. On the basis of the classic law of cubic equal ratio proportion, the ordinary least square (OLS) was applied to handle the tested data of 15 experimental conditions of the mine main fan, and the cubic polynomial formula of flowrate and power was developed, and the cubic polynomial formula between flowrate ratio and power ratio was developed as well as. The formula and its curve of flowrate ratio and power ratio shows that there should consist in the limiting value, and the characteristics of flowrate ratio and power ratio is non-equal. The non-equal formula supplied for the estimation method of energy-saving efficiency of a fan adjusted by variable frequency.