This paper considers the three-phase grid-connected inverter (GCI) as a MIMO system, and presents some new perspectives on the stability of GCI. It is found that with traditional Nyquist criterion (NC)-based method which ignores the off-diagonal components of impedance or admittance matrix, the stability results of impedance and admittance models are different. To make up this disadvantage, a simple stability analysis method based on system feedback ratio matrix (SFRM) is proposed. Instead of ignoring the couplings of impedance or admittance, the SFRM in the proposed method can be equivalent to diagonal matrix by adding a perturbation matrix into the system model, so identical results of stability analysis under impedance and admittance models are attained. Furthermore, to ensure the correctness of stability result, the boundary conditions of the proposed method are also provided. Theoretical analysis, simulation and experimental results show that compared with the state of art methods, the proposed SFRM-based method is of low computational complexity and high accuracy, and is more suitable for engineering application.