It is an important task for the prevention of voltage instability of the power system to identify the weak nodes of grid and take corresponding quick and accurate control measures. First of all

the static voltage stability L

q

and L

P

index are deduced on the basis of L index of voltage stability local index of original power system and through writing on the PI equivalent branch power flow equation model and also with consideration of existence principle of branch flow equation. Then

the deficiency of the L

q

and L

P

index is analyzed. The improvement index L

pqu

is obtained through writing branch voltage equation and considering modification of existence principle of branch voltage equation solution. Finally

the advantage and disadvantage of the proposed index under two kinds of branch equation are analyzed

the voltage stability L

u.p

index based on a kind of branch equation solution is proposed with combination of the branch flow equation and branch voltage equation so to keep them identify the weak branch of the system. In this paper

an actual power network at certain area of Xinjiang is taken as an example to analyze and verify the fast identification ccuracy and linearity of the proposed index to the weak point of power network and has a guidance significance of the dispatching personnel in fast and accurate identifying the weak point of the voltage of the system.