The modular multilevel converter(MMC) has emerged as a potential resolution in medium- and high-voltage applications, owing to its advantages, such as high efficiency and low distortion. However, the intricate structure of MMC with hundreds of distributed sub-modules per arm leads to complex internal dynamics, like circulating current and submodule capacitor voltage ripples. These internal dynamics can exert a significant impact on the output characteristics of the MMC, therefore, establishing a precise equivalent model stands out as a primary technical obstacle in the field. This paper aims to explore the influence of internal dynamics on the output characteristics of the MMC and subsequently analyze and assess the equivalent impedances of the AC side. The characteristics of the equivalent series impedances, which encapsulate the impact of capacitor voltage and circulating current, are rigorously investigated. Unlike existing models, this paper focuses on the effect of equivalent impedance on the performance of MMC, rather than overall impedance. As a result, the proposed model offers a more concise and targeted solution for analyzing internal dynamics in applications. Then, based on the equivalent model, the analytical formula of resonant frequency with a potential threat to the MMC operating stability is obtained. Ultimately, extensive simulations carried out in both the time-domain and frequency-domain validated the precision and effectiveness of the proposed model.