The use of bamboo charcoal (BC) was investigated as a filler in bamboo-plastic composites (BPCs) to achieve improved water resistance, physical-mechanical properties, and reduced hydrophilicity. The influence of the BC content and size on the water absorption, contact angle, density, and mechanical properties of bamboo flour/low-density polyethylene (LDPE) composites was tested. Scanning electron microscopy was used to analyse fractured and flat composite surfaces. The results indicated that the BC increased water resistance, achieving optimal results at 8% content. The BC particles that ranged in size from 60 to 100 mesh were more water-resistant than other sized BC. The water contact angle increased with an increase in the BC content or a decrease in the particle size. These results indicated that BC reduced the composite hydrophilicity and that the smaller BC particles improved this effect. The BC strongly connected with the LDPE composites, and the BC contents below 12% improved the flexural properties and increased the density of the BPCs. Also, the impact strength of the BPCs decreased dramatically with a decrease in the BC particle size. These results demonstrated that the integration of BC with BPCs resulted in strengthened water resistance and physical-mechanical properties and reduced hydrophilicity.