A flexible DC optimization control strategy is proposed to enhance grid interconnection across zones, addressing challenges such as inadequate supply capacity in layered and zonal grids, load imbalances, and low safety and reliability in the Southern Xinjiang region's power network. Firstly, based on the operational demands for grid security and economic performance, steady-state and transient optimization control objectives are established. Secondly, a comprehensive evaluation index is developed, covering load balancing, network losses, and voltage deviations. The optimal power of the flexible DC system is determined through a step-by-step approximation approach, ensuring optimal power flow control during the steady-state operation of urban power grids. A simplified model of the Southern Xinjiang urban power grid, featuring an outer 750 kV ring and an inner 220 kV ring grid structure, is developed for simulation. The simulation results validate the effectiveness and engineering practicality of the proposed method.