The project schedule was made according to the ptimistic time,the intervals of mission du-ration in the mission list were quantified unascertainedly, the unascertained completion time and itsreliability and cumulative reliability were obtained by use of the addition operation of unascertained rational numbers. The buffer time was calculated by the optimistic construction period and total project period when completion reliability was 90%,then the project schedule of the critical chain was established. The case study shows that the project schedule of the critical chain made by the theory of un-ascertained rational number and ptimistic time can relieve the effect of uncertainties and save the construction period compared with the traditional schedule.