Load characteristics are the main factor to affect voltage stability. In addition, load modeling reflecting the actual load characteristics has been a well—known difficult problem and unsolved so far, It is mainly due to the fact that the load composition, amount and characteristics are always changing. This paper introduces the slip into the equivalent impedance load model to analyze load characteristic, the varying slip is employed to indicate time-varying load characteristic precisely, and considering the dissimilar load behaviors, discusses node voltage, power during the changes of load characteristic, obtains voltage inflexion and power inflexion, and then analyzes the mechanism of power system voltage instability based on static voltage stability region. An example indicates the feasibility of the method.