Reasonable maintenance strategy can decrease power grid operation costs and enhance competitiveness of power grid. Therefore, according to the principle of system condition-based maintenance, an equipment imperfect maintenance model is built based on a modified Kijima II model, which considers electrical equipments' maintenance outage model and repair outage model and the effects of equipment maintenance costs on equipment reliability. What's more, a novel power system condition-based maintenance model is established. This model is solved by the genetic algorithm. Considering that the system condition-based maintenance can mitigate the conflicts between system operation and maintenance, this paper mixes equipment's imperfect maintenance risks and its effects into system risk evaluation, which pins down the system risks and obtains a more economic power system condition-based maintenance model. The proposed model is testified effectively based on a distribution transformer and the IEEE-RTS79 system.