In this work, flow-induced motions (FIMs) of an elastically mounted circular cylinder with roughness strips in the transition between Vortex-induced Vibrations (VIV) upper branch and galloping are investigated by solving the 2-D URANS equations in combination with the Spalart-Allmaras turbulence model with a finite-volume discretization method. The vibration of the PTC-cylinder is constrained to the transverse direction only. The vibration amplitudes, vibration frequencies, lift coefficients and the vortex structures are discussed, and the driving mechanism is also revealed. The numerical results correspond well the experimental data. The results show that during the transition from VIV upper branch to galloping, the amplitudes of the cylinder increase continuously with the reduced velocity increasing, but the frequency ratios of the cylinder decrease slowly, and the vibration finally reaches to the high amplitude, low frequency galloping stage. The maximum amplitude ratio in galloping can reach 3.88D. The phase differences between lift force and displacement are all close to 0° in the transition from VIV upper branch to galloping, and the lift force can promote the vibration to galloping efficiently. Meanwhile, the increase of shedding vortices, the differentiation of vortex intensity and the complication of the vortex pattern intensify the pressure fluctuation on the surface of the cylinder, which can intensify the fluid-structure interactions.