In this paper, a nonlinear ultrasonic modulation method is developed to detect early fatigue damage in aluminum alloy structures. Seven aluminum alloy specimens with different fatigue damage degrees are prepared by fatigue test. An experimental system is designed for performing nonlinear ultrasound modulation detection of fatigue damage in aluminum alloy specimens. A single piezoceramic transducer is used to emit two superposed sinusoidal waves with different ultrasonic frequencies. The higher frequency is chosen as a non-integer multiple of the lower frequency so as to be distinguished from super-harmonic responses. The dependencies of the nonlinear modulation response on the frequency and amplitude of excitation signals are explored to select appropriate signal excitation parameters. A nonlinear modulation index (NMI) is defined as the amplitude ratio of modulation responses and linear responses, which is used to evaluate fatigue damage of specimens. Experimental results indicate that the NMI increases monotonically with the degree of fatigue damage, and it can be used to quantify the accumulation of fatigue damage in specimens. The proposed nonlinear ultrasound modulation method facilitates the detection of fatigue damage and further assessment of the severity of the damage in aluminum alloy structures.