A simulation approach was proposed to analyze the spectra of Doppler signals in local expansion artery, which can provide a useful guidance for detecting the formation and growth progress of the aneurysms using the Doppler ultrasound technology and forecasting the size of aneurysm. Firstly, by solving Navier-Stokes equations, the velocity distributions in the vessels with various expansion degrees are calculated. Secondly, power spectral density (PSD) of the Doppler signals is estimated according to the relationship of velocity profiles and power spectral density. Finally, Doppler signals are generated using cosine-superposed methods. The results show that the spectra of Doppler signals are similar to those found in practice. Therefore, the proposed approach is useful for simulating the spectra of Doppler ultrasound signals.