In order to study the new methods to improve the aerodynamic and acoustic characteristic of multi-element airfoil, this paper presents the lift characteristic of two-element airfoil by wind tunnel test and numerical simulation with the upper surface of flap in smooth and rough situation, respectively. The test was accomplished in NF-3 low-speed wind tunnel of NWPU. The experiment model was two-dimensional model with 87 flush orifices. The smooth and rough situations were achieved by filling sound-absorbing porous material in upper surface of flap and using plastic membrane to overlap it or not. The numerical simulation was carried on the two-dimensional simulation of ANSYS Fluent with the unstructured mesh and k-ω SST turbulence model. The result shows that when sound-absorbing material is exposed, the low and medium frequency noise is eliminated. But the rough surface causes the decrease of average velocity in slot, the increase of fluctuation velocity and the reducing of the lift coefficient of airfoil. Thus, it is essential to keep the smoothness of upper surface of flap. Moreover, in multi-element airfoil wind tunnel test, in order to guarantee the quality of data, the surface cleaning must be done in time to maintain the surface smooth.