The reactive power reserve of AC-DC transmission networks is not yet within the focus of current research. An optimal reactive power reserve dispatch method is established for the AC-DC transmission network. At first, deduced from the AC-DC power flow equations of AC-DC networks, an effective reactive power reserve definition is proposed for the AC-DC transmission network by the generator control sensitivities of both voltage and reactive power. Based on which, an optimal reactive power reserve dispatch model is proposed under several operational constraints. To solve the above optimal power flow problem with two operational statuses, a decoupling solution is proposed. The simulation results on IEEE 39 bus system manifest that, the proposed definition of reactive power reserve is able to represent the voltage level of AC-DC networks, and the proposed model is able to enhance the security of power system via the increments of the reactive power reserve and voltage stability margin. This work is supported by Shanghai Engineering Research Center of Green Energy Grid-connected Technology (No. 13DZ2251900).