For the quasi-concentric cavity with large scale (ten/ens meters) used in high average outputpower far-infrared FEL, the short-pulse effects make the numerical simulation must involveevolution in both of space-time which means huge quantity of calculating. Decreasing quantity of For the quasi-concentric cavity with large scale (ten / ens meters) used in high average outputpower far-infrared FEL, the short-pulse effects make the numerical simulation must involvevolution in both of space-time which means huge quantity of calculating. quantity of