The invention provides a distribution network reactive power optimization method based on network decomposition and region coordination, comprising the following steps: decomposing a distribution network system into multiple distribution sub-network regions; establishing a reactive power optimization decomposition-coordination model with an equation as a constraint; iteratively solving initialization data of each distribution sub-network region in the reactive power optimization decomposition-coordination model; establishing a target function containing a Lagrange multiplier for each distribution sub-network region; and coordinating the adjacent distribution sub-network regions in exchanging boundary node information, and updating the data of each distribution sub-network region. According to the optimization method provided by the invention, the large distribution network is decomposed into multiple interconnected regions, the matrix dimension is reduced, and less memory is occupied; only boundary variable interaction is needed between the regions, and the utilization rate of communication resources is high; the degree of coupling between the regions is low, which facilitates parallel processing and improves the computing speed; and decomposition and coordination between the regions are independent of optimization algorithms, and the regions can choose an optimization algorithm applicable to their own characteristics.