The tonal noise, radiated by interaction between stator and rotor, is the main noise source of the subsonic axial fan. A rapid method to predict the noise is significant to the parameter design in early stage of the fan low radiation noise design. Therefore, a semi-analytic prediction formula of the power of radiated tonal noise of the interaction between stator rotor cascade was derived, by considering the interaction of stator and rotor blades based on the cascades response function theory, and by simulating the rotor wakes using empirical rotor wake model. The applicability of the formula was verified by comparing with the results of the existing experimental model and computational model from NASA. Based on NASA experiment model, the effect of rotor-stator spacing and stator blade parameters on tonal noise of fan was analyzed further. And the results show that increase of the rotor-stator spacing and lean design of stator blades suppresses the tonal noise caused by the interaction between stator and rotor effectively .