A mid-frequency oscillation accident recently happened in a practical MMC-HVDC project for offshore wind farms in China, which led to the outage of the system. The aim of this paper is to reveal the mechanism and characteristic of the mid-frequency oscillation in this practical system. The small-signal state-space model of the whole system was established by considering wind turbine generator, offshore MMC and submarine cable. Then, the eigenvalue-based method was used to analyze the mechanism of the mid-frequency oscillation. The interaction between the phase-locked loop (PLL) of the wind turbine generator and the current inner-loop control of the offshore MMC was found to be the key reason for generating the mid-frequency oscillation. Finally, the theoretical analysis has been verified by time-domain simulation results.