Based on renewable cellulosic resource, sisal pulp (SP) was prepared from wasted sisal fiber (SF) by the sulfate pulping method. The impact of the content of SP on the mechanical properties of phenolic composites was studied. For comparison, the reinforcement effect of SF, aramid pulp (AP), and glass fiber (GF) was studied. The microscopic structure and properties of the composites were characterized by polarized optical microscopy (POM), scanning electron microscopy (SEM), Dynamic thermomechanical analysis (DMA), and mechanical testing. Results show that the flexural strength and impact strength of SP-reinforced phenolic composites maximized at 125.6 MPa and 11.09 kJ/m(2), respectively, with 25wt% of SP. Compared with SF-, AP-, and GF-reinforced phenolic composites, the flexural strength of SP-reinforced phenolic composite has increased by 76.41, 25.35, and 14.29%, respectively, and impact strength increased by 49.26, 65.03, and 132.01%. POM illustrates that SP, SF, and AP subjected to a large shear during the roll milling process, and thus the fibers are cracked into finer microfibers. The results indicate that those microfibers and interfacial interaction affect reinforcement effect significantly.