Superconducting YBa₂Cu₃O_7−x (YBCO) bulks have promising applications in quasi-permanent magnets, levitation, etc. Recently, a new way of fabricating porous YBCO bulks, named direct-ink-writing (DIW) 3D-printing method, has been reported. In this method, the customized precursor paste and programmable shape are two main advantages. Here, we have put forward a new way to customize the YBCO 3D-printing precursor paste which is doped with Al₂O₃ nanoparticles to obtain YBCO with higher thermal conductivity. The great rheological properties of precursor paste after being doped with Al₂O₃ nanoparticles can help the macroscopic YBCO samples with high thermal conductivity fabricated stably with high crystalline and lightweight properties. Test results show that the peak thermal conductivity of Al₂O₃-doped YBCO can reach twice as much as pure YBCO, which makes a great effort to reduce the quench propagation speed. Based on the microstructure analysis, one can find that the thermal conductivity of Al₂O₃-doped YBCO has been determined by its components and microstructures. In addition, a macroscopic theoretical model has been proposed to assess the thermal conductivity of different microstructures, whose calculated results take good agreement with the experimental results. Meanwhile, a microstructure with high thermal conductivity has been found. Finally, a macroscopic YBCO bulk with the presented high thermal conductivity microstructure has been fabricated by the Al₂O₃-doped method. Compared with YBCO fabricated by the traditional 3D-printed, the Al₂O₃-doped structural YBCO bulks present excellent heat transfer performances. Our customized design of 3D-printing precursor pastes and novel concept of structural design for enhancing the thermal conductivity of YBCO superconducting material can be widely used in other DIW 3D-printing materials.