Abstract: Design of flare gas relieving system is a very complicated process, involving the calculation of each pipe back pressure and Mach number. This paper introduced the characteristics and application of Aspen Flare System Analyzer simulation software. Following the API specification requirements, the software can calculate the parameters such as pressure, temperature and flow rate at the beginning and the end of each pipe sections in flare system, so as to facilitate the identification and modification of the unreasonable design of the pipe diameters. In this paper, how to apply this software to review the flare system in an engineering project was demonstrated by taking an oil field surface facilities development project in the Middle East as an example. Calculation results of high pressure flare system showed that the back pressure and Mach number of some pressure safety valves' tailpipe were too high. With the modification advised by software, the pipe sizes can satisfy the relieving gas requirements in operation, which ensures security of the system. Result indicates the value of this software in pressure safety valve selection and flare-net design.