Abstract: Fiber aspect ratio has a significant effect on the mechanical properties of concrete. Because the change in aspect ratio is essentially the change in fiber diameter and morphology, the existing researches mostly change the aspect ratio gradient by changing the diameter or the cross-sectional shape of polypropylene(PP) fiber (including coarse and fine PP fiber), resulting that the variables are not unique. An experiment was carried out on the effects of the aspect ratio of coarse PP fiber (d=700 μm) and fine PP fiber (d=80 μm) on the mechanical properties of concrete. The mechanical properties of coarse and fine PP fiber reinforced concrete were analyzed. The results show that the slump of coarse and fine PP fiber concrete decreases first and then stends to be stable with the increase of the aspect ratio. The compressive strength, flexural strength and tensile strength tend to increase first and then decrease with the increasing aspect ratio. There is an optimum aspect ratio which is 42 for the 700 μm coarse PP fiber, and 200 for the 80 μm fine PP fiber. In addition, the macromechanical fitting calculation theory was proposed to analyze the influence of the aspect ratio of coarse PP fiber on the flexural strength of fiber concrete, so as to enhance the predictability and controllability of the test results. The analysis of the frictional bonding mechanism of the coarse and fine PP fibers in concrete was carried out, and the specific factors affecting the frictional bonding force were mastered.