Abstract: The stability analysis of the back-to-back converter is the fundamental basis of safe and reliable operation for the power electronized new-type power system. This paper establishes the detailed model and studies the small-signal stability of the three-phase symmetrical back-to-back asynchronous grid-connected system based on the modular multilevel converter. First, a frequency-domain algorithm is proposed to fast solve the steady multiharmonic variables using Newton-Raphson iteration. Then, an impedance model based on the multiharmonic linearization is established fully considering the frequency-coupling effect, whose accuracy is verified by comparing with the frequency scans over the wideband. Finally, the generalized Nyquist criterion is applied to explain the small-signal unstable cases, which reflects the necessity of theoretical models including the converter dynamics of both ends. Discussions are also given on the relations between stability criteria on AC/DC interconnection ports.