Abstract: In order to quantify the terahertz radiation of objects, the terahertz signals in blackbody radiation were measured. All measurements were conducted in vacuum circumstance, with liquid-nitrogen refrigerating to avoid any outside stray light. Meanwhile, those terahertz signals were focused on a detector by Cassegrain telescope. Afterwards, the output signals were further analyzed by a lock-in amplifier, and were compared with each other in different time constants. Results suggested that, with a constant temperature of 323 K, the measurements before and after liquid-nitrogen refrigerating were different with a discrepancy of about 3 v. Moreover, the time constant also had a great influence on the experimental results, with 3 s most appropriate in this study. However, due to the complicated noise sources and high noise levels, the radiation energy of the blackbody was too weak, and made the temperature resolution of the detection systems very low.