Abstract: The behaviour of rotor blades moving through the wake flow coming from stator departs from one which is designed by the potential flow theory, for example, the lift force on the blades is fluctuating. Hence the total pressure in flow is changeable. Based on the vortex theory, the present paper deduces an expression of the fluctuating lift force on cascade moving through a small shear flow, and presents a simple computational method by using the suggestion given in Ref.[4]. Comparison with the experimental results of Ref.[9] is satisfactory. Our method helps the rotor designer to adjust the design parameters selected.