A fusion inhibitor used to treat HIV infection, has an amino acid sequence as follows: X-SWETWEREIENYTKQIYKILEESQEQQDRNEKDLLE-Z, wherein X is either an amino group or -X1-X2, wherein X1 is an imino group, and X2 is either an acetyl group, a hydrophobic group, or a macromolecular carrier group; Z is a carboxyl group or -Z1-Z2, wherein Z1 is a carbonyl group, and Z2 is either an amino group, a tert-butyloxycarbonyl group, a hydrophobic group, or a macromolecular carrier group. This inhibitor has strong inhibitory activity against HIV infection.