The impedance analysis method is common for understanding the small signal stability of new energy grid-connected systems. In impedance modeling works, the three-phase symmetry of the grid side AC system is assumed, and the port impedance characteristics of the converter can be described using a second-order matrix. However, under asymmetric conditions, the converter exhibits more complex frequency coupling characteristics, and the model is no longer applicable. This paper firstly considers the sideband effect of a pulse width modulation（PWM） modulation and the frequency aliasing effect of sampling link, phase-locked loop dynamics, DC voltage control, positive and negative sequence current inner loop control, and other influencing factors, and establishes a small signal model of grid-connected converter suitable for three-phase asymmetric conditions. Then, a method for equivalenting a multi-input-multi-output （MIMO） system to a single-input-single-output （SISO） system under asymmetric operating conditions is proposed, and the correctness of the theoretical analysis is verified through PSCAD/EMTDC. Furthermore, the asymmetry of the grid-connected converter system is defined and the influence of control parameters on the system asymmetry is analyzed. Finally, the stability of grid-tied systems under asymmetry operation condition is analyzed based on the maximum peak Nyquist criterion.