Polysulfone (PSf) membranes were prepared by the wet-phase-inversion using 1-hexadecyl-3-methylimidazolium thiocyanate [C16mim][SCN] as an new additive. Scanning electron microscope was utilized to visualize cross-sections of the membranes to gain more better understanding the influence of [C16mim][SCN] on the pore-forming. Moreover, Membrane-forming mechanism using ionic liquid as a pore-former was also discussed. The results showed that ionic liquid [C16mim][SCN] had restrained the demixing of the casting solutions and has the ability of the pore-forming, higher than those of its analogues of PEG400. With increase of concentration of ionic liquid [C16mim][SCN] in casting solutions, the structures of the membranes changed from asymmetric finger pores to the spongy-finger-macrovoid structure of the pores. Compared with PEG400 as a pore-former,[C16mim][SCN] has the pore-forming ability to the membrane at lower concentration of [C16mim][SCN] in the casting solution. Especially, at the 4:76 ratio of [C16mim][SCN]/NMP, the prepared membrane has the asymmetric finger-pores structure, with the 2~6μm pores close to surface layer and the 10~24μm pore for its sublayer. Its retention rate of PEG10000 and solution flux are 97.1% and 48.7 L• h-1• m-2. Meanwhile, ionic liquid can be a plasticizer, according the thermal properties of the membranes.