Abstract: A model reduction for discrete-time singular systems especially the non-casual subsystems was investigated. A system transformation should be done first for the original system to enable the reduction problem to become merely as a simplification of the non-casual subsystems, thus getting a true model reduction of singular systems, i.e., retaining all the original non-casual attributes. Then, the controllability and observability were analyzed and they were weighed up in their corresponding states using the magnitude of Hankel singular values. The results showed that both the controllability and observability in the state corresponding to the smaller Hankel singular values were weakened. Based on it, a new reduction criteria was put forward with a new model reduction algorithm given. A numerical simulation illustrated the effectiveness of the proposed algorithm.