In this study, bamboo scrimber composites (BSCs) was prepared using two types of bamboo units and a phenolic (PF) resin, and the effect of heat treatment on water resistance and mechanical properties was investigated. The results showed that heat treatment significantly improves dimensional stability but degrades mechanical performance owing to a change in chemical composition and decreased porosity. Furthermore, the water resistance increases with an increase in resin content. The WAR and TSR of BSC-T-II reached 3.74% and 3.72%, respectively, at 25 wt% resin content. Heat treatment modification does not significantly improve the flame retardancy of BSCs, and future studies should consider treating bamboo with flame retardants. The BSCs have the potential to replace nonrenewable synthetic materials for outdoor use by optimizing heat treatment, bamboo unit processing, and resin content.