This paper proposes a novel admittance model for the stability analysis of grid-connected voltage-source converters (VSCs) which features a symmetric structure. On this basis, the grid-connected VSC system can be represented by a reciprocal two-port circuit to explore the oscillation mechanism of the system. Furthermore, the grid-connected VSC system can be modelled as a single-input single-output (SISO) system rather than a multi-input multi-output (MIMO) system. We analytically evaluate the influences of the VSC control parameters and the power factor on the stability of the grid-connected VSC by employing the Nyquist criterion and the stability margin. In addition, we demonstrate that the VSC's current output is in inverse proportion to the system's gain margin. Further, an analytical expression of the current limit constrained by small-signal stability is derived in closed form so as to calculate the admissible current output. All of analysis results are verified by simulations based on Simulink and hardware-in-the-loop (HIL) platform.