Compared with the ground focusing mechanism, the working environment of space focusing mechanism is more harsh and complicated. Hence the accurate prediction of its reliability is indispensable. However, it is unrealistic to obtain sample parameters of reliability analysis through repeated entity tests, due to the limitation of project cost and timetable, which lead to many existing model cannot be applied directly. We focus on a screw guide type space focusing mechanism, and propose a new time varying reliability probabilistic model that can characterize the reliability of discontinuous motion mechanisms with discrete multi-load. Its primary uncertainties and failure modes are evaluated quantitatively by failure mode effects analysis. The total damage of intermittent operation is treated as accumulative effect caused by each focusing work. And main parameters of the model are obtained by analytical calculation of mechanism wear reliability model. Finally, an integrated theoretical method are constructed and verified in this article, and the time varying reliability of proposed focusing mechanism are also discussed.