In the mould manufacturing projects, there are large numbers of uncertainties, such as change of customer demand, change of drawing, bad quality of products etc., and they often lead to numerous task reworked. Because of the random task reworking, the complete time of the project is uncertainty. Based on the critical chain project with task reworking, the paper carried out the research on setting of project buffer. We presented a new idea that the project buffer was calculated based on safety time and possible reworking time, and proposed a method to calculate the buffer of project reworking based on Discrete-time Markov chain. The problem that the classical method to setting critical chain project buffer is infeasible concerning task reworking was solved efficiently. The proposed method can set the project buffer more reasonable.