The techonology of power electronics has widely used in power generation, power transmission and power utilization of modern power system. At the same time, power system is expanding and integrating new energies sources such as wind power, photoelectric power, hydroelectric power, etc., in a large scale. As a result, the randomness of power system becomes increasingly evident. Under the new background, new models of the power system are necessary for its research. The new background of power system is summarized as power electronics, randomness and smart grid in this paper. It is pointed that the new needs for power system modeling may include accurency, wide-area and variation. Then the new trends of power system modeling are foreseen, i.e. equivalent modeling, on-line distributed modeling and hybrid time and frequency domain modeling.