A method is presented to monitor the weak voltage load node groups with Thevenin equivalents. First, continuation power flow solves voltage-load parameter curves of the load nodes. Then weak voltage nodes are selected with voltage magnitude variation under similar load variation rate. With considering about the relation of low voltage level and voltage collapse, voltage magnitude variation rate is chosen as the observation information of weak voltage load nodes grouping. The weak voltage load nodes are fuzzily clustered according to their similarities and simultaneously the geographic factors are taken into consideration. This paper tracks network side Thevenin equivalents on each typical node of weak voltage load node group and analyzes the problems of Thevenin equivalents computing based on local measurements. Therefore, a power flow data based simulation method to track Thevenin equivalents is proposed, which can observe the voltage stability margin of weak voltage load node group. The simulations on IEEE 118 system show the proposed scheme is effective.