In order to investigate the vortex-induced vibration characteristics and vibration mechanism of two square cylinders with small spacing in different arrangements， numerical simulation on vortex-induced vibration of two square cylinders in tandem， parallel， staggered arrangement is carried out at Reynolds number Re=100 with the spacing ratio of 2 and the mass ratio of 3. The amplitude and frequency of vortex-induced vibration （VIV） response of two square cylinder at Ur=1-30 is studied and the lift and drag coefficient are obtained to understand the characteristic of the aerodynamic coefficient. The wake structures of two square cylinders in different arrangements are analyzed. The results show that the vibration amplitude of downstream cylinder in tandem，staggered arrangement is much larger than that of single square cylinder while the vibration of upstream cylinder is suppressed （except the case of θ=60°）. The amplitude curves of two cylinders in parallel arrangement are almost identical， with the vibration amplitude larger than that of single square cylinder. The wake galloping phenomenon of two cylinders is observed in all arrangement，which make the cylinders maintain a lager amplitude as Ur exceeds the resonance range. The CD of two cylinders in different arrangement abruptly increases in the resonance range and keeps constant beyond the resonance range （except the CD of downstream cylinder at θ=30°）. In the resonance Ur range， the variations of CLrms with Ur of two cylinders are related to the A* while that keeps constant beyond the resonance range. There are three wake mode of two square cylinders in tandem arrangement. The wake structure of cylinders at θ=30° is similar to that of cylinders in tandem， while it is in a state of chaos beyond the resonance range.As θ=60° and 90°， the shear layer of two square cylinders sheds disorderly at lower Ur while the vortex sheds individually from each cylinder at higher Ur.