The numerical model of the unsteady flow field of ducted propellers is based on CFD (computational fluid dynamics). By applying the numerical model, the unsteady flow field of the ducted propeller with the fracture in different positions of a certain blade is simulated and its unsteady hydrodynamic performance is numerically analyzed. By extracting the fluctuating pressure data of the duct inner wall monitoring points,the fluctuating pressure-time oscillogram of ducted propellers is obtained, and then the spectrum is obtained by FFT transformation of the oscillogram. A blade fracture fault diagnosis technology of ducted propellers, which combines oscillogram analysis method with spectrum analysis method, is put forward by analyzing and studying the oscillogram and the spectrum.