The outgassing of packaging materials is one of the direct threats to the vacuum acquisition and maintenance of vacuum electronic devices, so its accurate measurement is of great significance to improve the quality and life of devices. However, the background leakage and outgassing of the sample chamber have a non-negligible impact on the measurement results. The outgassing rate that can be measured by traditional testing devices is generally high, and it is difficult to accurately measure the vacuum outgassing rate of packaging materials with low outgassing rates. In order to solve this problem, we develop an apparatus for measuring the vacuum outgassing rate of materials based on constant conductance method and symmetrical measurement structure, and propose a background outgassing compensation method to extend the lower limit of the outgassing rate that can be measured.