The Modular Multilevel Converter (MMC) technology has matured significantly, and its startup process, which is essential for normal system operation, can be divided into two phases: uncontrolled charging and controlled charging. After the uncontrolled charging phase, the submodule capacitor voltages have not yet reached the rated level, requiring specific control strategies for voltage elevation. Existing methods mainly leverage the controllable characteristics of submodules during this phase, regulating voltage by adjusting the number of blocked submodules. It is important to note that the charging process must strictly avoid risks of overvoltage and overcurrent. To address this, this study proposes a novel constant-current pre-charging control strategy: DC-side phase unit blocking is implemented to simulate a constant DC source for the converter, while AC-side arm unit blocking is used to create a three-phase symmetrical AC source with a constant amplitude. This strategy combines ease of operation with broad applicability across systems. Simulation results based on MATLAB/Simulink verify that the proposed method effectively achieves constant-current startup control for both the DC and AC sides of the MMC.