In this study, varying contents of ultrafine bamboo-char (UFBC) were introduced into PLA/bamboo particle (BP) biocomposites as new reinforcements to improve the mechanical, thermal, and morphological properties of the biocomposites. The new strategy was aiming to realize the synergistic effects of reinforcement and toughening of poly(lactic acid) (PLA) composites through a simple method without surface modification and other additives. The maximum tensile strength, modulus, and elongation at break of 45.20 MPa, 540.50 MPa, and 7.53% were reached at 5.0 wt% UFBC content, which were slightly lower than those of pure PLA. The maximum modulus of elasticity of the ternary biocomposites was 5316.1 MPa at 5.0 wt% UFBC content, which was approximately 2 times higher than the pure PLA. Impact strength reached a maximum value of 38.56 J/m when the UFBC content was 5 wt%, and improved by 376% compared with pure PLA of 7.88 J/m. Meanwhile, compared with the PLA/BP binary composite of 20.50 J/m, it improved 88%. A concrete-like microstructure system was achieved (i.e., cement, sand, and rebar corresponding to PLA, UFBC, and BP, respectively).