There is open fire in the equipment of the modified Claus process for sulfur production after recovering the elemental sulfur in the sour gas of purification, gasification and synthesis processes of  Shouyang Chemical Corporation Ltd.of Yangquan Coal Group, so it is necessary to ensure no air at the equipment inlet. In order to better control combustion equipment and prevent unsafe working conditions, a safety instrument system, SIS interlocking system, was designed, which not only ensures the stability and safety of sulfur recovery device, but also meets the requirements of environmental protection.