The development of offshore wind power base (OWPB) offers advantages in coordinated layout and unified planning. However, planning methods suitable for small-scale offshore wind farms are no longer adequate for OWPB at the ten-gigawatt level. Transmission system planning for such bases involves partitioning the wind power base, locating offshore booster and aggregation stations, optimizing transmission routes, and selecting grid connection points. To address these complexities, this paper proposes a planning method for grid integration of OWPB based on a coded planning matrix. First, the improved K-means cluster method is introduced to partition the OWPB. Second, the improved gravity center method is adopted to locate the offshore collector and booster stations, considering varying cable transmission loss and capacities of wind turbines. Then, aiming for optimal economic performance, a planning model based on a coded planning matrix is established to achieve coordinated planning of partitioning, station locating and sizing, cable selection, and grid connection point optimization. Finally, simulations are conducted using a real OWPB for verification, and the results show that the proposed method achieves faster computation and more economical planning outcomes than traditional planning methods.