Solid state reaction interdiffusion of Fe-Ti multilayer modulated films was investigated by means of dynamic in situ X-ray diffraction. With this method, the measurement of the X-ray modulation peak intensity was relatively accurate and the shift in X-ray modulation peak position was also observed. The interdiffusion coefficients of Fe-Ti modulated films were calculated at different temperatures. Plots of the interdiffusion coefficients versus time at different temperatures were given, According to the variation rate of D vs.t, the activation energy Q relative to viscosity relaxation was calculate to be 8.3±0.2 kal/mol.