Load model plays an important role in the power system stability analysis and digital simulation. In the composite load model there is a load controlled by thermostat controller, such as industrial heating, electric water heater and electric space heater. With the proportion of the thermostatically controlled load (Thload) increasing, the influence on the power flow distribution and static voltage stability is more significant. Aiming at these problems, this paper analyzed the impact of this load on the static voltage stability. Firstly, the model of Thload is introduced and its mechanism and characteristics are set forth. Then this paper has made analysis and comparison of the static voltage stability between Thload and other loads based on the foundation of power system analysis toolbox (PSAT) in MATLAB‥, and the impact of the parameters of the Thload on the small disturbance voltage stability was discussed. At last, according to the results of small disturbance method, the effect of Thload and other loads on the electromechanical mode of single-machine infinite-bus system was analyzed comparatively.