The vortex-induced motion (VIM) of the four square columns platform at subcritical Reynolds number (Re = 5400 ~ 30000) was studied by model test and numerical simulation. The effects of flow direction angle and reduced velocity on the key characteristics of VIM are analyzed, and the coupling mechanism among yaw, sway and surge is emphatically revealed from the aspects of motion response amplitude, vibration frequency and force characteristics. The research shows that the flow incidences have an important influence on the force and motion response of the four square columns platform, but the influence degree on each motion direction is different, especially on surge. under flow incidences of 0, 30 and 45 degrees incidence, no matter in the process of model test or numerical simulation, there is no "lock in" phenomenon in the strict sense of sway and surge, but a special phenomenon that is not common in the existing literature is found. When the reduced velocity Vr is ranged from 2.78 to 6.48, the vibration frequencies of sway and surge are locked to their natural frequencies, but there is no obvious resonance phenomenon, which may be caused by the special shape of the square column and the phase difference between yaw, sway and surge. There is no frequency locking phenomenon in yaw, and the vibration frequency and amplitude always increase with the increase of reduced velocity. Furthermore, under the above three flow incidences, the vibration frequencies of sway and yaw are always consistent, which proves that there is a high coupling relationship between the two motions, which further shows that yaw is also dominated by vortex induced lift force. At 15 degrees incidence, the combination of vortex-induced motion and galloping was observed in the three motion directions. Bounded by the reduced velocity of 6.48, it showed vortex-induced motion at low reduced velocity and galloping at high reduced velocity, which may be related to the increase of low-frequency component of vibration at high reduced velocity.