In a distributed generation system, the phase-locked loop (PLL) has a crucial impact on the transient stability of the grid-connected voltage source converters (VSCs). This paper discusses the instability mechanism of PLL and the interaction between the PLLs in the parallel grid-connected inverter system. The dynamic characteristics of PLL considering the coupling of grid impedance and line impedance in weak grid are given in this paper. It is proposed that the movement of the steady-state operating point of the PLL will bring it closer to the unstable point under the disturbance. Due to the nonlinear characteristics of the PLL, the phase portrait is applied to distinguish the transient stability of the multi-PLL system. It is shown that the transient stability of system is degraded due to the PLL interaction in a multi-VSC system. The effects of inverter injection current, grid impedance and line impedance are discussed. Finally, the conclusions of this paper are verified by simulation comparison.