Increasing renewable energies bring more fluctuating power outputs, and make operating reserve become more important for maintaining the system balance. Regulating flexible loads to provide operating reserve has been widely accepted as a promising alternative, while the communication network will face enormous challenges due to the frequent data transmission and explosive data volume of large-scale distributed loads. To address this issue, this paper proposes a coordinated optimization framework of power-communication coupling networks for dispatching large-scale flexible loads to provide operating reserve. A power-communication equivalent model is established to couple the regulation power and transmitted data from flexible loads. The data nodes and branches in the communication network are formulated equivalently with power nodes and branches in the power network. On this basis, considering spatially and temporally dynamic power-communication coupling networks, the power flow and communication flow are coordinately optimized to minimize the regulation costs and communication costs. The proposed scheme is validated based on the 5-bus and 118-bus power-communication coupling networks. Numerical results illustrate that the coordinated optimization framework reallocates operating reserve capacities and decreases the regulation cost of power-communication coupling networks.