Abstract: The modeling and stability analysis of modular multilevel converter（MMC） under symmetric conditions has received extensive attention. In practical engineering, MMC might be in asymmetric conditions, such as asymmetric inductance of the bridge arm and asymmetric voltage on the AC side. The admittance modeling and stability analysis under asymmetric conditions need to be further studied. At present, considering the admittance model of the system with multi-frequency coupling and the quantitative admittance dimensionality reduction methods are the difficulties in the stability analysis of asymmetric conditions. Therefore, this paper mainly establishes the MMC admittance model suitable for asymmetric conditions, and studies the matrix dimensionality reduction method suitable for small disturbance stability analysis under MMC asymmetric conditions. The dimensionality reduction method is based on the vector norm of admittance matrix. It is suitable for “black box” system and various asymmetric conditions, and can quantitatively evaluate the dimensionality reduction error of the admittance matrix with each order. Taking the Guangxi-side model of Luxi back-to-back asynchronous grid-connection project of China as a case, PSCAD/EMTDC electromagnetic transient simulation is used to verify the accuracy of the proposed model and the applicability of the proposed dimensionality reduction method under various asymmetric conditions.