Dewar is the key component of an infrared detector and its quality has the direct influence on the reliability of the whole thermal imaging system. In order to detect the vacuum characteristics of a Dewar better, a set of equipment for checking a Dewar for its holding time of LN2 is designed in this paper. In this set of equipment, the performance variation of a Dewar when a device is in vacuum can be determined by testing the parameters such as the open-circuit voltage of the device and the volatilization of LN2 . In this paper, the heat leakage of a Dewar is calculated in theory and a lot of experiments are carried out. By comparison of the experimental data with the theoretical calculation, it can be found that this set of equipment has a better measuremnet precision and its expected design aim is achieved.