DC collection and transmission is a new trend for large scale offshore wind farms. In this paper, the output voltage of the wind farm is promoted by series connecting the wind turbines. Then, the wind power is collected by shunt connecting the wind turbine group. VSC-HVDC is a traditional method to connect offshore wind farm to onshore AC grid. However, big voltage variable ratio dv/dt is the main problem. Hence, multi-level converter is required for the process from DC to AC. Modular multi-level converter is suitable with its good performance. So, simulation model for DC-based offshore wind farm with modular multi-level converter is developed in MATLAB/Simulink. Meanwhile, the pulse modulation and system control scheme is designed. Simulations are done to verify the presented trial.