The work aims to study the effect of fiber surface modification on the crystallization, thermal stability, dynamic mechanical performance, dimensional stability and water absorption of the composites. The fiber surface was modified by alkali, alkali+maleic anhydride and alkali+KH550. The PLA (polylactic acid)/Flax composites were prepared by mixing the PLA through the melting extrusion. The introduction of flax fibers with modified surface made it easier for PLA cold crystallization, and made the crystalline structure more compact and perfect. The dimensional stability of PLA/Flax composite was better than that of the PLA. The incorporation of fibers improved the water absorption of PLA, but lowered its thermal stability. Through fiber surface modification, the energy storage modulus of PLA/Flax reduced. The crystallinity and dimensional stability of composites prepared by the mixture of the fibers treated by alkali+KH550 and the PLA are the best, which provides certain experimental and theoretical basis for the preparation of the high-performance PLA/Flax composite.