Abstract: A device for measuring a thermal conductivity of high-temperature and high-pressure liquid is provided and includes a probe and a liquid flow channel, the probe radially penetrates through the liquid flow channel, both ends of the probe extend out of the liquid flow channel and are connected to a control system via wires; the control system includes a power supply, a voltmeter, an ammeter, a thermocouple and a flowmeter, which are used for energizing the probe, measuring voltage at both ends of the probe, measuring current flowing through the probe, measuring temperature of and a flow velocity of the liquid, respectively. A measuring method is further provided. The device and the method for measuring the thermal conductivity of high-temperature and high-pressure liquid utilize a principle of heat balance and characteristics of liquid sweeping across a circular tube for measurement when the liquid enters a fully developed section.