On the basis of the presented two new concepts of trisingular equivalence class and trisingular set a new approach based on the rough set for horizontal decomposition of inconsistent decision tables is proposed. The new approach, just as the existing approach does, can decompose an inconsistent decision table into two decision tables, i.e., a consistent decision table and a complete inconsistent decision table. But the consistent decision table decomposed may be greater than that obtained by using the existing approach, thus causing less information loss. It is noteworthy that the new approach may be used to decompose parts of complete inconsistent decision tables, whereas complete decision tables can not be done by using the existing approach.