Abstract: Based on the dispersion equation and the displacement field expression of ultrasonic guided waves, ultrasonic guided wave amplitude spectra have been studied numerically for a unidirectional carbon fiber sheet. The gray images of ultrasonic guided wave amplitude spectra that can straightforwardly characterize the degree of fatigue in composite sheets are obtained. The numerical results show that, with the tension-tension or bending fatigue loading, there is a monotonic relationship between the average gray value of gray images of ultrasonic guided wave amplitude spectra and the change rate of equivalent Young‘s modulus or equivalent shear modulus. It is concluded that the ultrasonic guided wave amplitude spectra may be used to effectively characterize the early stages of fatigue damage in composite sheets.