In order to obtain the preventive control scheme of voltage stability under unstable contingency, a preventive control method to avoid voltage instability based on local curve fitting is presented. Based on the study of characteristics of voltage stability saddle node bifurcation point, a method to calculate the load margin sensitivity without the left vector is proposed based on local curve fitting equation. These sensitivity information of loading margin with respect to control parameters are used to establish a linear programming model for preventive control, and then the control scheme to avoid voltage instability is given. Tests with IEEE-57 bus system indicate that the sensitivity is calculated accurately and the operating point is pulled back to voltage stability security region effectively.