Grid forming(GFM) converters can support the AC-side terminal voltage relying on its inner voltage control loop. To improve output current response under the transient process and enhance the disturbance rejection capability of the voltage loop, the Grid Current Feedforward(GCFF) control is widely equipped in the voltage control loop of GFM converters. However, GCFF samples the grid side current and it aggravates the coupling of grid dynamics and converter dynamics which makes the voltage dynamics of GFM more complicated and may affect stability. In this paper, a terminal voltage dynamic model of a grid-connected GFM converter system for stability analysis that contains the effect of the GCFF algorithm is proposed. It reveals that the GCFF control forms a positive feedback loop in terminal voltage dynamics and reduces the system stability margin under large GCFF coefficients and low grid impedance conditions. The theoretical analysis is well verified by the simulation results.