In order to investigate the mechanism of wake-induced vibration of the suspension bridge hangers, and to verify the applicability of quasi-steady assumption in the research of the wake-induced vibration, a series of wind tunnel tests for two parallel circular cylinders were carried out to measure the aerodynamic derivatives of the downstream cylinder. Second, a theoretical analysis model of wake-induced vibration of the downstream cable was developed on the basis of the unsteady theory, and the Runge-Kutta method was adopted to numerically solve the motion equations to obtain the responses of wake-induced vibration. The results show that obvious oscillations of the leeward cable take place in the spatial region of 5.2≤X≤5.6 and 1.1≤Y≤2.1, and the vibration frequency of cable is slightly less than its natural frequency. A positive work within a period is always done by the aerodynamic stiffness force if large amplitude vibration of wake-induced vibration takes place, which indicates that the aerodynamic stiffness force is the key factor to evoke the wake-induced vibration of the leeward cable. Moreover, the results based on the quasi-steady theory show small variation when compared with those based on the unsteady theory, and the wake-induced vibration mechanism obtained by the two theories is consistent.