Abstract: In this paper,the region splitting of multigray level picture is directly brought about with a optical step filter. It is discussed in theory that a step filter processes a multigray picture with hard and soft boundaries to realize the region splitting of the multigray level picture. The discussion shows that with the step filter. The boundary enhancement in the region will appear when the noncontinuous gray grade (a hard boundary) and the epoch emerge in the multi-gray picture .