In order to enhance observation accuracy of the spindle induction stator flux and decrease observation error which is influenced by temperature, frequency and magnetic path in operation process, a full order observer was used to observe stator flux. The change of stator flux observer which adopted u-i model and low pass filter estimation model observation error was analyzed when the parameters of motor changed through the mathematical model in the stationary coordinates. A method to determine the gain matrix of full order observer was presented. The results showed that the error changed with the changes of motor speed and torque. On the premise of assuring stability of the observer, according to different speed ranges, choosing the proper observer pole assignment, the method can be used to enhance the observation accuracy of stator flux at high speed, and it is also validated in the Matlab/Simulink condition. It is shown that it can improve observer accuracy of the induction motor stator flux effectively.