Abstract: In order to solve the problem of poor mechanical properties of hemp straw fiber/cementitious composite, a modified method of hemp straw fiber/cementitious composite by polyvinyl alcohol (PVA) emulsion was proposed. After optimizing the particle size and mixing amount of hemp straw fiber, PVA emulsion was blended with hemp straw fiber and cement to form the modified hemp straw fiber/cementitious composite. The effects of PVA emulsion with different mass ratios on the flexural strength, density, specific strength and flexural toughness of modified hemp straw fiber/cementitious composite were studied. The modification mechanism was revealed by moisture content, water absorption and fourier infrared spectroscopic analysis. The results show that when the particle size is 1700 μm and the mixing amount is 12%, the hemp straw fiber has the best reinforced effect on hemp straw fiber/cementitious composite. With the mass ratio of PVA emulsion increasing, the density of the modified hemp straw fiber/cementitious composite declines and the flexural toughness improves gradually. When the mass ratio of PVA emulsion is 4.8%, the flexural strength and specific strength of the modified hemp straw fiber/cementitious composite are increased by 17.17% and 20.50%, respectively, compared with the unmodified hemp straw fiber/cementitious composite. The interface between straw fiber and cement in hemp straw fiber/cementitious composite is improved and the negative effect of hemp straw fiber on hydration reaction is alleviated.