The invention provides a three-phase reactive power optimization method of an active power distribution network. The three-phase reactive power optimization method comprises the steps of inputting trend data, and constructing a three-phase active power distribution network reactive power optimization model in consideration of an interphase mutual inductance of the branch circuit in a manner that the least value of a power distribution network active network loss is used as an objective function; converting the three-phase active power distribution network reactive power optimization model to a semi-definite programming relaxation model, and solving according to a semi-definite programming method; processing a discrete variable according to a sensitivity analysis method and a branch-and-bound method, and obtaining an iterative value; substituting the iterative value into the semi-definite programming relaxation model, and solving again until an optimization model optimal solution which contains the discrete variable. The three-phase reactive power optimization method of the active power distribution network realizes reactive power optimization calculation of a three-phase active power distribution network and furthermore realizes accurate expression for different-type load conditions in the power distribution network.