Superconducting YBa2Cu3O7.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.