Large-scale offshore wind farms usually use HVDC transmission based on the Modular Multilevel Converter (MMC). However, the system is likely to cause instability because of some irrational control parameters while the wind farm is connected to the MMC. To solve this problem, a small-signal model of a hybrid-based wind farm which contains a Doubly-Fed Induction Generator (DFIG) and Permanent Magnet Synchronization Generator (PMSG) direct-drive wind turbines interconnected with MMC is developed. Then the eigenvalue method is selected to study the oscillation mode of the system. The study shows that there is an interaction between the phase-locked loops of wind farms and the voltage control loop of the MMC, and it also forms a weakly damped oscillation mode. The proportional coefficient of the MMC voltage outer loop control has a great influence on the dominant oscillation mode. The larger the proportional coefficient is, the more stable the system is. When the proportional coefficient is too small, it will lead to low-frequency oscillation of the system. All the results are verified by simulation in PSCAD/EMTDC. The research results will provide a reference for the reasonable selection of control parameters in the engineering of wind farm transmitted power through MMC-HVDC. This work is supported by National Natural Science Foundation of China (No. kz17010138).