Similar large signal synchronizing instability that is common in traditional power system also exists in voltage source converter (VSC) dominated power system, which is increasingly reported and investigated. In this paper, the large signal instability of phase locked loop (PLL) synchronized VSC connected to weak ac grid is investigated. First, the influence of high grid impedance on PLL's dynamics is explored and an additional feedback loop is found to be introduced, which deteriorates the performance of PLL. Then an analysis model that is similar as the rotor motion model of synchronous generator (SG) is developed. Based on the developed model, equal-area method is employed to carry out the large-signal stability analysis. The large signal instability is found to be mainly resulted by the following two factors. One is the nonexistence of equilibrium point, which is similar as that there does not exist intersection between the mechanical power input curve and electromagnetic restoring force curve in SG grid connected system. The other is related with the transition process resulted by insufficient decelerating area, which leads to that the system cannot stably go through the transition from initial point to the existing equilibrium point. The proposed analysis not only contributes to revealing the physical mechanism but also provides guidance for the future improvement measures.