In the present study, submicron and micron-sized pinus sylvestris char (PS) was applied for the modification of polylactic acid (PLA) and the effect of the properties of PS on the mechanical, thermo-mechanical, and crystallization kinetics of PLA were investigated. As demonstrated, PS could enhance the toughness of the PS-PLA composites by forming interfacial "bridges" to facilitate stress transfer due to its stable lamellar structure and interaction with the PLA matrix. The tensile strength of PLA-PS composites increased by 98% and the bending strength increased by 25% compared to that of pure PLA. The addition of PS significantly increased the energy storage modulus (increased 22%) and loss modulus (increased 30%) of PLA, but led to a decrease in the crystallinity and thermal stability of the composite. In the practical utilization for 3D printing, PS-PLA composites possess good tensile strengths (41.5 MPa), allowing for smooth printing of complex products. This work provides an efficient strategy to modulate the thermodynamic and mechanical properties of PLA matrices by regulating the viscosity of the composites and modulating the interfacial strength with the addition of proper amounts of biochar.