The invention discloses a low-temperature radiometer for calibration of solar irradiance, and a direct-connection thermal structure therein, relates to the field of sensor on-orbit optical radiation measurement, and solves a problem that the direct connection of a low-temperature platform and a heat sink in the terminal structure of a conventional radiometer causes that the temperature stability of the heat sink cannot meet the measurement requirements of a low-temperature radiometer. The radiometer comprises a Brewster window, a 77K radiation cold shield, an aluminium housing, a radiometer internal thermal structure, and a refrigerating machine. The radiometer internal thermal structure specifically comprises a stray light elimination diaphragm, an absorption cavity, a heat sink, a temperature control heat sink, and a heat link. In an actual optical radiation measurement test, the temperature control heat sink is provided with an electric heating wire for temperature adjustment in a lining manner, so as to adjust the temperature stability of the heat sink through controlling the temperature of the temperature control heat sink. The radiometer improves the heat transfer efficiency of an absolute radiometer, and reduces the time constant in a measurement process. The arrangement of the temperature control heat sink enables the temperature of the heat sink to remain in the range of 1 sigma.