Abstract: With the development of technology, phased array antennas have been increasingly applied on spaceborne platforms and the electromagnetic performance requirements of spaceborne phased array antennas are also higher and higher. The high electromagnetic performance of antennas has strict requirements for structural stiffness, lightweight design and efficient heat dissipation etc. Moreover, spaceborne phased array antennas are generally installed on the outer surface of spacecraft, so the space environment such as deep cold space and solar radiation has a significant impact on the temperature of the antenna. The thermal design of a spaceborne phased array antenna is studied in this paper. The heat dissipation scheme, heat pipe selection and thermal control measures of phased array antenna are analyzed based on the thermal design input. The rationality of the thermal design is verified through simulation and experiments.