In a solar hybrid system, the intermittent solar radiation seriously effects the solar-to-electricity conversion. In this paper, the energy-level mechanism between the concentrated solar heat and the thermal cycle was discussed. The system analysis was taken on a 200 MW coal-fired power plant hybridized with solar heat at approximately 300 °C, where the middle-temperature solar thermal energy was used to preheat the feed water before entering the boiler. With changing solar radiation in typical days, the solar share, the work output and the net solar-to-electricity efficiency of this solar hybrid system were evaluated. The net solar-to-electricity efficiency would be increased by 3–7% points compared to that in a solar-only power plant. An off-design parallel configuration of this hybrid system was proposed, achieving the net annual solar-to-electricity efficiency of 18%. It would expected to be an attractive approach to develop the scale-up mid-temperature solar thermal power technology in the short and midterm.