The present invention relates to a power transmission system optimization method on an offshore wind farm cluster scale. The power transmission system optimization method comprises the steps of: (1) setting quantities of offshore wind farms to be established, wherein each quantity corresponds to a scheme; (2) taking the lowest total construction cost as an objective, and adopting a dual-layer control method for optimizing offshore substation positions, offshore substation capacities, an electricity collecting network and a power transmission system topology in each scheme separately; (3) and selecting the offshore substation quantity, the offshore substation positions, the offshore substation capacities, the electricity collecting network and the power transmission system topology corresponding to the scheme with the lowest total construction cost as the optimal result. Compared with the prior art, the power transmission system optimization method proposes to establish offshore public substations for a plurality of wind power owners to use, and put the offshore power grid planning and onshore overhead line extension planning into the same system, performing unified planning and unified establishment according to the offshore wind power planning situation combined with onshore power grid structures, and achieves the optimal cost and efficiency from the whole.