Polyester has many superior characteristics such as high strength, durability, and good wash-and-wear properties. But it is not comfortable to wear. Modifying polyester fiber with animal and plant protein was seldom reported, and the fastness of protein was not good enough because there are only few hydroxyl groups on the surface of polyester fibers. In this study, sugar ester(SE) which had both hydrophobic aliphatic chain and hydrophilic hydroxyl groups were encased into surface layer of polyester fiber at high temperature and pressure. The hydrophobic part of sugar ester could be inserted into the polyester fiber, and the reactive hydroxyl groups would stay on the surface of polyester fiber. Then soybean proteins could be grafted on the surface of polyester fiber with sugar ester glycidyl ether (SEGE) as crosslinking compound. Surface morphology and structure of the fabric were studied by SEM、FT-IR、XRD、DSC. The results showed that the surface of polyester fiber was covered by SE and soybean protein overlay film, the encased sugar ester and grafted soybean protein fiber had little change on structure, and the proper grafting temperature was 100°C, time 30min. Encasing and grafting technology is a promising method for modification of polyester fiber.