It is important to get the accuracy aerodynamic character data for designers during the airplane design phase. The wind tunnel test with scaled model is one of the main ways. The wind tunnel's finite Re number limit the assessment of the transonic cruise drag, so we have to make the Re number effect correction for the drag of wind tunnel test. This article analyses the tolerance of wind tunnel drag measurement, and presents the wind tunnel test data basis for Re number effect correction. The constitution of transonic cruise drag is analyzed . The correct method of the Min-Drag and Lift-Induced-Drag is discussed respectively, and the result is verified to get a reliable engineering method.