Traditional outage model for the power equipment usually focus on the behavior of the equipment under random factors, and the availability of the power equipment in system analysis is usually confined to the steady value. However, this model may be inaccurate in the short term analysis, where the transient process of availability has not ended yet. Furthermore, the power equipment in the short term analysis might be influenced by both random factors and deterministic factors, yet the impact of deterministic factors cannot be completely reflected in the traditional outage model. Based on the above issues, a Markov-based transient outage model is proposed in this paper, which describes the deterioration and repair process of an equipment. Both the corrective maintenance and preventive maintenance are concerned in the model. The preventive maintenance in the model is considered as deterministic event, in which the start time and duration are both scheduled. Meanwhile the corrective maintenance and the unexpected failure are modeled as random events. The transient state probability and availability of equipment under preventive maintenance is derived. The effect of deterministic events on the availability of equipment is analyzed on numerical tests. The proposed model can be used in the short-term reliability assessment and maintenance scheduling in actual systems.