The invention provides a system and a process for desalting recycle-water of iron and steel combined sewage by a bi-membrane method, which belong to the technical field of treatment and recycling of sewage. The system comprises a cooling tower, a raw water tank, a micro-flocculation filter device, a flow by-pass activated carbon filter kettle, a laminated filter, an ultrafiltration system, an ultrafilter, a reverse osmosis system, and a reverse osmosis desalination water tank. The system and the process are used for solving the problem of desalination of the recycle-water of sewage by the bi-membrane method for steel enterprises. In the system and the process, the pretreated water is treated through the ultrafiltration device by bi-membrane desalination to meet the requirement that SDI is less than or equal to 3, and the water is subjected to desalinating treatment through the reverse osmosis device. The system and the process treat the recycle-water of the iron and steel combined sewage by a mode combining preoxidation, micro-flocculation filtration, flow by-pass activated carbon filtration and self-cleaning filtration, thereby greatly improving the operating stability and the service life of the bi-membrane desalination system, effectively realizing sewage reclamation and 'zero drainage' of the sewage, and having important practical value on the desalination of the recycle-water of the iron and steel combined sewage.