[en] Highlights: • Modified CFs were firstly used as reinforcement phases of composites made by selective laser sintering (SLS). • The interfacial adhesion between fillers and matrix in SLS were firstly discussed. • The oxygenic groups on the surface of CFs were thermally decomposed during the sintering process. • Treating CFs at 400 °C after HNO₃ modification can decrease the porosity of sintered composites. • The practical application problems of modified CF/PA12 composite powder in SLS were resolved. This work aims to study the surface modification and reinforcing effect in carbon fiber (CF)/nylon 12 (PA12) composites of CFs, and more importantly, resolves the practical application problems of modified CF/PA12 composite powder in selective laser sintering (SLS). CFs were surface modified by nitric acid (HNO₃), the surface changes of CFs were characterized by SEM, FT-IR and XPS. Initially, the effects of the experimental factors on the properties of composites were investigated, and the CF/PA12 composite powder prepared with optimized technological parameters was subsequently adopted in SLS experiments. The sintering behavior and relationship between mechanical properties and porosity of sintered specimens were also discussed. The results indicated that the oxygen functional groups on HNO₃ modified CFs were partly thermally decomposed and released as gas in the process of laser sintering, which caused high porosity and poor mechanical properties of sintered specimens. Treating CFs at 400 °C (approximately the laser sintering temperature) after HNO₃ modification was proposed as a solution. This treatment preserved part of the modified effect and reduced the porosity of the composites at the same time. Compared with the unmodified CF/PA12 sintered specimens, the tensile strength, tensile modulus, flexural strength, and flexural modulus were all increased to different degrees.