It is important to develop accurate component outage models for reliability evaluation in power system. However, the traditional models neglect the impacts of deterioration and maintenance of equipment, making the estimation error increase with the operating time. Aimed at this problem, a time-varying outage model based on aging characteristics and imperfect preventive maintenance is proposed, in which the virtual age and improvement factor are used to characterize the effects of maintenance and the Weibull model is adopted to reflect the aging trend of failure rate. The proposed model is applied to the reliability evaluation of a modified IEEE RTS-79 system, the reliability evaluation results of different maintenance strategies with different cycles and costs are given in detail and compared with that of traditional model. All the results show that the proposed model is more accurate in power system reliability evaluation and can reflect the actual complex maintenance. This work is supported by Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20110141110032).