Abstract: Mission-availability of inertial confinement fusion (ICF) laser facilities is considered as an important factor of facility construction. Based on basic models for mission-availability, processes of the facility performing tasks are analyzed with their types and a model diagram of mission-time for indicating time-limits of a mission and its repair presented, and then a mission-availability evaluation model for a single mission is given. Utilizing curve-fitting and goodness-of-fit test methods, and considering different impact on next missions and its probability, a mission-availability evaluation model for multiple tasks of the facilities is constructed. And then, the facility mission-availability evaluation methods and its steps are definitely given to provide a general model and method for systematic research of ICF laser facility mission-availability evaluation. Failure data of SG-Ⅲ prototype facility during operation are processed and analyzed by the use of the evaluation approach, to verify that the approach and the model are correct and economical. The results will be directly used for reliability engineering construction of SG-Ⅲ laser facility.