Sisal fibers were pretreated by alkali and alkylation, and then mixed with polylactic acid to make sisal fiber-reinforced polylactic acid composites, respectively. The architecture and aspect ratio of sisal fibers before and after mixing, the micro-morphology of fracture surface, and the properties of composites were investigated. The results showed that surface treatment greatly impacted the fiber fracture behavior. Shearing fracture occurred to the untreated sisal fibers and alkali-treated sisal fibers, which led to a sharp reduction in the length of the fiber, while the sheartear facture occurred to the alkylation-treated sisal fibers and resulted in obvious decrease in both length and diameter of alkylation-treated sisal fibers. The fiber aspect ratio of sisal fibers was affected first by the pretreatment method, then by the processing temperature, and the sisal fiber content. Furthermore, the mechanical properties of reinforcing fibers and sisal fiber-reinforced polylactic acid composites were influenced fundamentally by the pretreatment method.