Abstract: Phloem fiber (or Bast fiber), a kind of non-woody plant fiber, is widely used to reinforce composites due to its good mechanical properties and eco-friendliness. In the cell wall of phloem fiber, abundant cellulose microfibrils with the helical structure are embedded in an amorphous matrix composed of hemicellulose, pectin, and lignin. The variation of the cellulose microfibril angle forms a highly ordered hierarchical structure of the cell wall. The assembly structure and compositions at different scales are of great significance for mechanisms and principles of the excellent mechanical performance of phloem fiber. This work summarized the structural characteristics of phloem fibers represented by hemp and flax at the tissue level, cell wall level, ultrastructural level, and molecular level. The emphasis was focused on the underlying interactions at different levels which generated the special mechanical behavior of the phloem fibers during the axial stretching process. Finally, the existing problems were pointed out, and the development trends of future research were prospected. The extracted concepts may provide new ideas for improving the utilization of phloem fiber and serve as inspiration for biomimetic applications.