The hydrodynamic stability of a liquid wavy film isimportant to guarantee its function for efficient heat and mass transfer.However, the stability has been affected by various factors. Under a gasflow at the gas-liquid interface, the stability is obviously influenced bythe interfacial shear at the boundary condition. The evolution equation ofthe surface waves under shear effect is derived with the integral approachbased on the boundary layer theory, and the effect of interfacial shear onthe stability under different conditions is illustrated with the presentmodel. The theoertical results show that film flow tends to be unstableunder the cocurrent shear, and the effect of countercurrent shear plays apositive role under lower Re and a negative role under higher Re. Thecritical wave number and critical wave velocity increase under the cocurrentshear and decrease under countercurrent shear. Therefore, the effect ofinterfacial shear on critical wave velocity is clearly different underdifferent Reynolds number.