When the slit-type buckets are applied to the orifices in a high-arch dam, the nappes trajectory distance is basic to the design of the plunge pool. The hydrodynamic characteristics of nappes from orifices with slit-type buckets in a high-arch dam were studied in this paper. And based on the geometric similarity and law of free falling body movement, the method of nappes trajectory distance calculation was put forward. The results were in good agreement with those of model test.