In order to investigate the influence of vacuum environment on the measured value of solar irradiance absolute radiometer (SIAR) and its auxiliary value to the world radiometric reference (WRR), and improve the tracing accuracy of on-orbit solar irradiance measurements to WRR and consistency of measurements of different on-orbit instruments, the variation of cavity tempreture response and its effect of SIAR in vacuum environment are measured and estimated. Since the absence of air convection in vacuum, the time constant of SIAR cavity temperature response becomes longer and the equilibrium temperature becomes higher in the same heating power with that in air, thus the responsibility of SIAR cavity is significantly reduced.The radiometric measurement results of the same target in vacuum and air environment show a relative difference of 0.15% between the measured irradiances since the variation of time constant and responsibility, which causes 0.15% changes in the auxiliary value of radiometric measurements in vacuum to WRR.