To improve the stability of power system, this paper discusses the estimation issues of both power angle and unknown input for generator system, an observer-based estimated method is proposed to deal with the above problems. A fourth-order mathematical model of generator is derived from the motion and electromagnetic equations. The model is linearized by Taylor''s formula, where the remainder produced by model linearization and model uncertainty caused by parameter variation is regarded as the unknown input of generator system. A state transformation is used to decompose the generator model, so that the model is decomposed into two parts, containing unknown input item and no unknown input item. The observer of these two parts are designed respectively, gain matrices of them are obtained by method of pole placement and Lyapunov stability theory. The method of observer-based power angle and unknown input estimations for generator system is verified by the Matlab simulation, and this method presents characteristics of short response time and high accuracy of estimation, so that fast and accurate estimation of power angle and unknown input is realized. This work is supported by National Natural Science Foundation of China (No. 61403129).