The main goal of voltage quality control (VQC) system is to ensure the qualified voltage and the balanced reactive power in substation, which is the important guarantee to improve power quality. According to the oscillation occurred in the VQC system of a substation based on seventeen-zone theory, the fixed value and control strategy were analyzed, and the control times of capacitor and the fixed value of reactive power during flat peak period were the main reasons of oscillation. So the control strategy of capacitor and the fixed value during flat peak period were adjusted and improved. Through continuous track inspection, the measures were proved effective, which can solve the problem of too many unqualified voltage points of bus line in substation and improve the qualified rate of voltage.