The integration of distributed wind farm leads to additional fluctuation of power flow in a high-voltage distribution network, which has become a major concern in automatic voltage control. This study proposes a slack optimal control method to determine the tolerance band of gateway reactive power (GRP) which takes into account voltage constraints, energy loss and excessive operations of control devices. Analysis of network characteristics shows that the GRP control band should be set according to both load levels and the outputs of distributed wind farm. Based on the analysis, slack optimal active loss difference strategy is proposed to set the slack optimal band of GRP. A slack optimal band matrix is defined and a partitioning intersection strategy is proposed to implement the differential setting of the slack optimal control band parameters of GRP. Simulation studies are conducted on a practical distribution network in China and the results show that the proposed method performs well in energy saving, control device regulating, voltage profile keeping, and reactive power balancing in a simple and practical way.