It is difficult to obtain wake galloping forces in wind tunnel experiments, but it is easy to acquire them by Fluent numerical simulations. In this paper, wake galloping responses of a spring-mounted downstream circular cylinder, with two degree-of-freedom in the wake of a stationary one, were simulated in staggered arrangement at streamwise spacing ratio L/D=2 and cross-stream spacing ratio T/D=1. In combination of structured meshing methods in ICEM and dynamic mesh techniques, the simulation was performed by the proposed unsteady fluid-structure coupling method, which embeds user defined code into Fluent by adopting the unsteady SST k-ω model for the 2D Reynolds-Averaged Navier-stokes (2D RANS) model. The study was carried out with reduced velocities V_r varying from 5 to 60 and Reynolds numbers Re varying from 2.4×10³ to 2.82×10⁴. Vibration responses obtained by the proposed unsteady fluid-structure coupling method were validated by experimental data and the aerodynamic forces at V_r=50 were compared to that calculated by the quasi-steady calculation method. Results indicate that the dimensionless amplitude A_y/D increases nearly linearly with the increase of V_r as a typical wake galloping phenomenon. The amplitudes have an excellent agreement with the experimental data. The wake depresses the random vortex shedding of downstream cylinder in vortex-induced resonance region. The trajectory of the downstream cylinder appears like a counterclockwise tilted oval with a clear directivity and self-limitation in the wake galloping region. Additionally, the quasi-steady calculation method has insufficient consideration on higher-order forces and vortex-shedding forces. The displacements obtained by the two methods have an excellent agreement. The self-excitation forces of primary four order frequency multiplication play a major role in controlling the displacement response undergoing wake galloping, indicating that the wake galloping is a kind of self-induced vibration.