Polylactic fiber is a new environment-friendly fiber and has wide application, while there are few studies on its characters and structures. The main content in present paper is to analyze element content and functional group as well as thermal properties. The elementary analysis method shows that the contents of C, H and O elements are 49.83%, 5.63% and 44.54%, respectively; it has good accordance with the theoretical results. Gas chromatography mass spectrum method proves that the functional groups of PLA fiber are lactide and acetaldehyde as well as 2,3-pentanedione, and whose total content is larger than 70%. IR spectroscopy depicts that there exist significant absorbing peaks by -C=O-, -C-O-, -OH- within 1000cm-1-1300cm-1, and -C=O- within 1000cm-1-1300cm-1. The absorbing peak of ultraviolet spectrum is 260 nm avelength. Crystallinity is 61.72% and orientation degree is 93.36% and 84.30% in Full Wave at Half Maximum and in Area calculations based on X-ray. These results will be greatly helpful in the qualitative analysis and indectification of Polylactic fiber.