Under normal circumstances, the frequency of islanding grid will drop because of power shortage after fault disconnection. However, voltage instability happened with increasing frequency under condition of large power shortage in practical power grid accidents. This paper uses the relationship between load power and basic electrical variables to research the transfer limits of apparent power and to analyze static operation point. And then voltage instability mechanism of the islanding grid after fault disconnection is analyzed from aspect of grid frame and voltage-power characteristic with simulation model. It is pointed out that load capacity will be limited if the load apparent power is close to transfer limit of islanding grid. Furthermore, restrictive mechanism between grid limit and motor load is analyzed by aspect of transfer limit of apparent power at each time section and to point out that reactive power of motor absorbed after short fault is an important cause of islanding voltage instability. And then it reveals that low electromagnetic power after voltage instability is the essential reason of low-voltage with high-frequency question. It provides reference for the operation mode arrangement of islanding grid and control of concentrated load shedding and under-voltage load shedding.