The growing research interest in diode rectifier unit-based modular multilevel converter (DRU-MMC) configurations has made it imperative to analyze their potential impact on the short-circuit current characteristics of future AC grids. Most existing short-circuit current calculation approaches are oriented toward DC-side fault scenarios, while systematic research on AC-side fault analysis remains incomplete. To address this limitation, this work first illustrates the operating mechanism of the MMC. The short-circuit current at the fault point is decomposed into two independent components based on the superposition theorem. These components are: the current injected by the MMC and the current originating from the AC system. This work further explores the regulatory mechanism by which dq-axis limiting and fault-ride-through current limiting shape the MMC’s output current and derives an analytical equation for its amplitude. Furthermore, the phase correlation between the MMC-injected current and the AC system current under symmetrical fault conditions is clarified, and the computational formula for the aggregate short-circuit current is established. Then, considering the influence of transition resistance, the proposed method is verified to be applicable to both symmetrical metallic and non-metallic faults. Then, the symmetrical component method is used to analyze the sequence component of asymmetric fault short-circuit current, and a negative-sequence suppression (NSS) strategy is introduced. At the same time, considering the influence of transition resistance, the calculation formula of asymmetric metal and non-metal fault short-circuit current is derived. At the final stage of the study, a two-terminal simulation model is constructed in the PSCAD/EMTDC simulation environment. Comparative verification confirms that the results derived from theoretical calculation are in strong agreement with the simulation outcomes. The approach introduced here offers a favorable combination of simplicity and precision, rendering it highly suitable for practical engineering use. It reliably determines the short-circuit current under various fault conditions, thereby supporting fault-current analysis and the coordination of protective relays on the AC side of the receiving-end MMC in a DRU-MMC system.