Slack time in flight schedule is redundancy time for flight turnaround． If slack time is too long，it is a waste of resource，such as planes，crews and time． Slack time is a significant method to absorb delay in actual operation． If it is too long， the propagated delay will be serious． A propagated delay tree with initial flight as the root was built to conduct dynamic research on the propagated process to solve the optimization of slack time． An optimization model of slack time redistribution was built，which has the advantages of certainty and linear easy to solve． Experimental results based on actual operation data of an airline show that the proposed model could reduce propagated delay by 28% without increasing costs while slightly changing the original fleet and crew schedule．