Abstract: In a compressor system, the blow down valve (BDV) is not only an important guarantee for safe operation of compressor, but also a major relief source, which is of great significance to the design of flare network and flare system. Compared with common pressure vessels, the calculation of blow down valve is unique as the inlet and outlet pressures of the compressor system differ greatly. Taking the medium-pressure (MP) compressor system in a project as an example, its stable temperature and pressure during shut down is calculated; then, the dynamic blow down block of Hysys software is used to calculate BDV to obtain the maximum instantaneous blow down rate; finally, the upstream and downstream pipeline sizes and BDV size are calculated based on the calculation principle. The calculation results indicate that, fire case is the main case in the sizing of BOV, if the compressor system is blown down, the blow down rate would decrease continuously and the properties of materials released change too.