This paper numerically investigated the effects of mass ratio on vortex-induced vibrations (VIV) of a two degree-of-freedom cylinder adjacent to a flat wall. The mass ratio is m* = 2, 10 and 20, the gap ratio is G/D = 0.6, the Reynolds number is Re = 100, and the reduced velocity is Ur = 3-12. In order to excite large amplitudes of VIV, the structural damping ratio is set as zero. It is found that, with the increasing mass ratio, the VIV of the near-wall cylinder is excited at a higher Ur with a smaller maximum vibration amplitude. Owing to the influences of the boundary layer, the vibration shows droplet-like orbits in most of the cases. However, when m* ≥ 10 and Ur is small, the vibration shows figure-of-eight trajectories. Moreover, for small vibration amplitude cases, the wake pattern show the 1S mode, but displays the C+S and 2S modes when the amplitude is large. Beyond hysteresis, the wake pattern is in the 1S mode for m* = 2 while in the steady mode for m* ≥ 10. It is further found that hysteresis in the vibration responses of the near-wall cylinder corelates with the bi-stability of the boundary layer reattachment on the cylinder surface. In the increasing-Ur cases, the boundary layer periodically reattaches on the upper surface of the cylinder, which excites larger amplitude vibration responses owing to the enhanced upper-side shear layer. However, in the decreasing-Ur cases, the boundary layer pass through the gap between the cylinder and the wall, and thus the lower-side shear layer and the vibration responses are suppressed.