To select a suitable time in the timetable of the university is in nature how to find a common free time set of classrooms, teachers and classes. In 1975 Itai and Shamir proved that the timetable problem also is a NP-Complete problem. There had been a lot of heated arguments among many scholars until then. We can divide the timetable problem into two layers to solve. At first we should select a suitable time slice, and then according to the time slice we proceed to schedule the classrooms. But classroom-schedulings are greatly different in various universities. Is this paper we will particularly deal with problem about how to select a suitable time-slice according to the safety decision-making principle. When we select a time slice for every course, we must consider the minimum influence upon successive courses. If a teacher will teach two courses which correspond two classes in a semester, when we select the time slice of the first course, we should try our best to select the time slice when the second class already has another course. In such a case when we select the time slice of the second course, the freedom of the selection would not be hindered by the first selection. If a class has two courses taught by two teachers in a semester, we must consider the problem in the similar way. As we select the slice in such a way, the possibility of collision would greatly decrease. Here we present the quantified description of the safety decision-making scheduling algorithm. In order to speed up scheuling process, when we input the course tuples, we put the course tuples with the same teacher and the same class together. We should only scan downward several courses, and then we can make our decision accordingly. Other courses are irrelevant In this paper we draw the algorithm flow-chart. The program has been run in the IBM-PC. It has turned out that to select a suitable time slice is fully probable.