This paper proposes a wide-area optimal control method of low-voltage reactive compensation equipment of radial distribution networks based on partial measurement data. First, the load power impedance moment (LPIM) method is used to partition radial distribution networks into several reactive power source control areas and program the optimal location of reactive compensators, and then the ideal reactive capacity of reactive compensators is determined; then, a wide-area reactive optimal model is created to control the capacitors online switching considering the target of reactive power partition balance and the limitation of distribution transformer capacity and the constraints of voltage margin. With reactive power partition balance of distribution networks, this method extremely increases the utilization rate of the low-voltage capacitors, reduces the active loss and promotes the voltage quality of distribution networks, so it has significant social and economic benefits. The effectiveness and practicality of this method are proved by an actual distribution network which has 390 nodes.