The performance and array layout of the tidal turbine are closely related to wake flow and wake vortex characteristics. In this paper, the flow field of a 2D vertical axis tide turbine (VATT) model is simulated with CFD. The numerical simulation shows that the wake of VATT can be divided into 3 regions with different vortex characteristics. Compared the wake vortex characteristics with the different inflow velocity, the number of blades and the tip speed ratio (TSR), respectively, it found that the wake is significantly affected by the TSR. At the low TSR, the apparent blade stall makes the vortex distribution of near wake field very disorganized; at the optimal TSR, the wake field is relatively stable; at the high TSR, the distribution of the tail vortex in the far field obviously similar to the Kármán vortex street. The change regulation between the circulation and the TSR is found by monitoring the circulation of the shed vortex. The formula of the vortex motion trajectory is fitted by tracing the shed vortex with the tracer particles released from the trailing edge.