Abstract: Vortex-induced vibration of cylindrical structures is a common phenomenon in engineering. If the distance between cylindrical structures is small, vortex-induced collision will occur. Vortex-induced collision is more serious than vortex-induced vibration on the fatigue damage of the structures. The immersed boundary method was used to simulate the dynamic boundary problem in the fluid which avoided the numerical instability problem when the traditional boundary-fitting method was used to solve the collision problem between solids. The finite element method was used to simulate the motion and collision of the cylinders. The lubrication model under fluid flow condition was established by data regression method. The vortex-induced vibration and collision of two side-by-side cylinders at different initial gap ratios were simulated numerically. The numerical results show that if the collision occurs, there will be a continuous collision. Multiple frequencies occur in collisions and the main frequency of vibration is higher than that without collision. When the two cylinders collide, the relative velocity is smaller than that of free flow. When two cylinders are close to each other, the transverse fluid force decreases with the gradual inclination of vortex ring separation angle. When the vortex rings between two cylinders start to influence each other and squeeze, the transverse fluid force starts to increase gradually. When the two cylinders start to rebound, a low pressure area is formed between the two cylinders, which changes the direction of the transverse fluid force and makes the two cylinders move close to each other again. This repetition results in the oscillation of transverse fluid force and cylinder velocity after collision.