By modern cybernetics, we can find out a state feedback for a linear time system, which does not change the controllability and stability of the system. But whether a state feedback for linear time system can be found out, which does ont change observability of the system or not, so far hasn't been discussed. The problem is studied according to the steady discrete time linear system, and a state feedback is found out, which does ont change the controllability, observability and stability, by relation of stability, observability and characteristic root, rank of matrix, of steady discrete time linear system; the method of how to construct the state feedback is given.