Abstract: In the small power supply enriching regions, because power flow fluctuates dramatically, the steady running voltage alternates so substantially that power delivery and supply are restricted seriously. To address this problem, it is significant for the reactive power filling system to research voltage control. On the basis of single-machine infinite-bus system model, the paper establishes firstly mathematical formulas of voltage deviation and real power、reactive power、resistance、reactance. Secondly, the paper carries out the sensitivity analysis, and estimates these relative sensitivity coefficients. Furthermore, the paper summarizes a law that the transformation ratio affects mostly the control point voltage and the control point voltage's change is sensitive mostly to the transformation ratio's change, presents the voltage control strategy which depends mainly on regulating transformation ratio. Finally, as a case analysis, the voltage control engineer about Qinkou 35 kV power transmission and transformation system testifies validity of the control strategy.