Abstract: Based on the simplified model of double-fed direct current motor system(DFDCMS),the controllability and observability of saturated nonlinear DFDCMS are analyzed,and the key states of saturated nonlinear DFDCMS are presented. Based on the subsection-linear model of the saturation nonlinear DFDCMS and Kalman filter(KF),the Kalman filter state observer is presented and designed to get these key states.On the simulation platform and the real system,the motion-actor-states observation results of three state observers,i.e.,the direct state observer with no filter(NF),the state observer with Butterworth filter(BF) and the state observer with KF,are compared in some different situations.The experimental results indicate that the KF state observer based on the model of saturated nonlinear DFDCMS can reflect running states of the testing system more effectively.