Under the trend of building a new power system, large-scale integration of off-shore wind energy into the transmission network presents new challenges to the safe operation of the grid. This paper proposes a two-level optimal partition approach for offshore-onshore interconnection system considering spatial-temporal electricity balance. First, a generalized electrical distance is proposed based on three indicators: impedance, line weight, and power interaction entropy, which can better reflect the strength of electrical connections between nodes in high-renewable scenarios. Second, a two-level optimization method is proposed to iteratively obtain the optimal solution. While the upper-level model is grid partitioning optimization with the minimum electrical connection index as its objective function, the lower-level optimal power balance model is to minimize the total offshore-onshore interconnection system cost. Finally, simulation analysis is conducted on the improved IEEE 24-bus system to verify the effectiveness and superiority of the proposed method.