Steady state and instantaneous availability have limitations in shot tasks.Based on a general model of mission-availability of inertial confinement fusion (ICF) facilities,a complete mission-availability model for disk amplifier system is proposed.A multilevel reliability block diagram is established.Equal relationship of expected accumulative failure number between Weibull distribution and exponential distribution is gained during each shot.Mission-availability model for disk amplifier modules with multi-failure modes of Xe-flash lamps and a method to evaluate mission-availability of the system with redundant modules are given successively by hierarchical modelling ideas.According to experimental data,the effectiveness of the method is verified and increasing trends of missionavailability of national ignition facility (NIF) and Shenguang (SG) facility are compared with each other,and then the suggested reliability index of Xe- flashlamps is obtained.The results provide designers a theoretical and methodological basis to optimize reliability design of the large ICF laser facilities.