Selective Laser Sintering (SLS) technology can be utilized to recycle residues from forestry and agriculture, thereby alleviating shortages of materials and reducing energy consumption by producing wood-plastic pieces for industrial application. The mechanical strength of wood-plastic SLS parts is low, which restricts the application of this technology. In this study, a novel type of sisal fiber/poly-(ether sulfone) (PES) composite was prepared using a polymer mixing method in order to improve the mechanical properties of SLS parts. Single-layer sintering method was adopted to determine the proper processing parameters. The mechanical properties of the parts with different ingredient ratios and different particle sizes of sisal fiber before and after post-processing were tested using a universal testing machine. The morphology was examined using scanning electron microscopy (SEM). Results showed that the mechanical properties of the printed parts were relatively enhanced; when the mixing ratio of composite powder was 10/90 wt/wt. In addition, the part fabricated by powder of particles size less than 0.105 mm (0.125 mm ≥ PS < 0.105mm) had the best mechanical strength. Moreover, the post-wax treatment significantly improved the strength of the parts, and the surfaces became smoother.