This paper proposes a fast reactive power flow control method for active distribution networks based on a distributed time-varying optimization algorithm. The objective of this method is to minimize the operational network loss of an active distribution network containing multiple microgrids by distributing the reactive power output of each node while ensuring the balance of supply and demand within the grid. The network loss cost model of active distribution network with multiple microgrid is established and the network loss function is constructed through sensitivity analysis, a distributed finite-time optimization algorithm is proposed based on the Lagrangian multiplier method, and the distributed time-varying optimization algorithm is employed to achieve optimal reactive power distribution among the microgrids, minimizing the overall network loss. Finally, simulation case analysis verifies the effectiveness of the proposed control method. The simulation case proves that the algorithm can achieve fast reactive power flow control and minimize the overall network loss.