A flue gas reheater, including: a heat absorption section; a first heat release section; a second heat release section; a control system; circulation pipes; a flow control valve; and temperature sensors. The first heat release section and the second heat release section are connected to the heat absorption section via the circulation pipes. A heat transfer medium is disposed inside the circulation pipes. The circulation pipes include ascending pipes and descending pipes connecting the heat absorption section to the first heat release section and the second heat release section. The flow control valve is disposed on a condensate pipeline entering the second heat release section. The temperature sensors are disposed in the first heat release section and the heat absorption section. The flow control valve and the temperature sensors are all connected to the control system.