When voltage source converters(VSCs) are employed as grid-interfaces, current feedback control in rotating synchronous reference frame(SRF) which generates from the phase-locked loop(PLL) is adopted for high-performance grid current injection. The cross-coupling between the dq-axes is usually decoupled by Inductor Current Feedforward(ICFF) control technique. However, the use of ICFF changes the dynamic characteristics of current control loop, and further interacts with the dynamics of PLL under the weak grid. This paper proposes a q-axis impedance model of VSC for stability analysis which considers the influence of ICFF control. It reveals that the ICFF decoupling technique reduces the system stability margin, even causes instability of VSC under weak grid condition. The above theoretical analysis is well validated by simulation results.