The coupled single-port circuit has been proposed for online voltage stability assessment based on wide-area measurements and grid equations. This circuit can explicitly reflect the influence of grid and load on voltage stability. However, it does not consider the impact of power generation. A novel Thevenin equivalent (TE) circuit considering the influence of source-grid-load on voltage stability is proposed in this paper. First, the impact of source-grid-load on the voltage of an individual load bus is divided into three parts according to the node-voltage equation. They are then decomposed into two components, respectively associated with the TE voltage and the TE impedance according to the TE circuit’s characteristics. Finally, an online identification method for the parameters of the proposed TE circuit based on wide-area measurements is developed. Compared with the coupled single-port circuit, the proposed method has the advantage of being able to consider the influence of power generation on voltage stability and being consistent in form with traditional TE circuit without requiring approximation. Case studies on the IEEE 118-bus system illustrate the accuracy and effectiveness of the proposed method.