Theoretical formula for discharge calculation of airfoil-shaped measuring flume in trapezoidal canal under free condition was proposed, based on the analysis on principle of circular cone and cylinder measuring flume which is presented by Hager. The coefficient of discharge correction is determined by relative head. A serial of indoor experiments about flume was carried out with 6 kinds of contraction ratios. Non-dimensional correlation between relative flow rate and relative head was developed according to experimental datum. The results show that there is a good relationship between relative flow rate and relative head. The maximum relative error of discharge calculation is less than ±3.5%, and the maximum submergence limit is 0.90. A preferable water measuring performances is obtained when the contraction ratio ranges from 0.372 to 0.585. 