With the increase of distributed generation integrated, proper allocation is important to enhance voltage stability of power system. To settle down the drawback of traditional participation factor based allocation method, a normal vector index based on voltage stability limit surface is proposed. The rationality of this index is proved at first. Numerical simulation of IEEE 39 system demonstrates the validity of the proposed method. Compared to the traditional index, normal vector index has a wider range of application because it can deal with the voltage stability critical point of saddle node bifurcation and limit induced bifurcation.