Abstract: During deep space exploration,the ability of being able to be repaired for an on-orbit unit can greatly improve the reliability of spacecraft system. However,researches on single-unit reliability with the influence caused by the on-orbit repair are really scarce. An availability model and its state equations of on-orbit repairable single-units are studied, and its transient availability formula is proposed. The availability with different repair rates of a repairable single-unit is calculated,and compared with a non-repairable one. A method for quick estimating the expected reliability at the end of mission by applying the relationship between repair rate and failure rate is presented. It is shown that the higher on-orbit repair rate,the higher reliability of a singleunit. The high on-orbit repair ability can also reduce the requirement on the reliability of a developing single-unit.