In order to fast locate the critical loads in a large-scale power system， and to analyze the impact of its uncertainty on the transient stability quantitatively， this paper systematically proposes a new method of the load model uncertainty's impact on the dynamic simulation of large-scale power grids. The first step of this method is to find the electrical centre during the disturbance， and rank the amplitude of load critical criterion from high to low. Based on this， which buses would be the most influential on the dynamic performance of the system can be determined， and the most critical loads can be preliminarily selected as well. The second step is to study the impact on system dynamic simulation from uncertainties of selected loads by combining with probabilistic collocation method（PCM）. Test on Hainan Power Grid verifies its practicability and validity in dynamic simulation. The proposed method can rapidly detect the most important load on the dynamic performance of the out-of-step generators for the operators， and quantitatively analyze the uncertainty of the dynamic response.