Aiming at the difficulty of Markov process and virtual age in describing systems under imperfect repair comprehensively, a model for a series system to determine its availability is established on the base of quasi renewal process. Methods for obtaining the mean time between failures and mean time between repairs are proposed. By introducing the operation time reduction factor and repair time increasing factor, availability parameters of systems under imperfect repair considering the aging factor are determined. According to maintenance costs per unit of time and average functioning time, the times of performing imperfect repair can be calculated. Finally, a control unit is used as an example. A traditional method about availability is applied to verify the effectiveness of the proposed method under imperfect repair. And imperfect repair times are calculated under different conditions including maintenance costs and operation time, which can offer suggestions for maintenance decision making.