The invention discloses a frequency control method of an offshore wind power integration multi-terminal flexible DC transmission system. When a shore-based converter station exits, asynchronous power grid transient frequency deviation drop and wind farm additional pitch angle control are realized through reasonable configuration of the controllers of a converter station in a power grid at a receiver side and through wind farm additional virtual inertia control, and asynchronous power grid steady-state frequency deviation is eliminated eventually. Thus, the frequency characteristic of adjacent asynchronous power grids is improved, and the influence of failure of a converter station on the operation safety and stability of other shore-based asynchronous power grids is reduced. By using the method to analyze the exit of a shore-based converter station of a large-capacity offshore wind farm integration multi-terminal flexible DC transmission system, the designed frequency control strategy can improve the frequency stability of a shore-based AC system.