Bolted-joint structures are prone to loosening under vibration, which affects the structural integrity. This paper presents a bolt looseness recognition method based on the sub-harmonic resonance analysis.The bolted-joint structure was simplified to a nonlinear single-degree-of-freedom (SDOF) model, and the multi-scale method was used to explain the sub-harmonic resonance. Experiments were performed on an aluminum beam with a loosened-bolted simulating the damage. Two surface-bonded piezoelectric transducers were used to generate continuous sinusoidal excitation and receive corresponding wave signals. The experimental results demonstrated that the sub-harmonic components appeared when subject to the excitation frequency twice of the natural value, which can be effective on bolt looseness damage detection.