For preemptive scheduling, the discrete scheduling has the same capability as the continuous scheduling. However, most work previously done has been focused on the continuous scheduling. In this paper, a discrete analysis approach Empty-Slots method is discussed and applied to fixed priority scheduling for constraint-deadline task model. Then a new schedulability test using the Empty-Slots method is presented. Compared with classical schedulability tests for continuous scheduling, this test is more convenient to be used and more efficient to tackle practical problems.