Printed circuit boards (PCBs) are usually required to be integrated and miniaturized by a variety of applications, such as 5G microstrip antenna and millimeter wave radar, which require the used dielectric substrate materials with a low dielectric loss (Df) and a high dielectric constant (Dk). The composite substrate prepared by filling dielectric ceramics into organic polymers has good machinability and superior dielectric properties, which is regarded as the most promising solution. However, the physical and chemical indexes, dielectric properties of the ceramic filler and its compatibility with organic resins are three key factors affecting the comprehensive properties of organic/inorganic composites. In this paper, a micron-spherical filler with a high sphericity and a high crystallinity was prepared by a special spheroidization technology with titanium dioxide material with superior dielectric properties(i.e., Dk ≈110, Df ≈ 0.001, dielectric constant temperature coefficient (α) ≈ -700×10-6 ℃-1). The compatibility between the filler and organic resin is improved after the surface modification of TiO2 filler. Also, the dielectric constant temperature coefficient of TiO2 filler is regulated by Al2O3 doping. The composite substrate filling with 20% (in mass) Al2O3 exhibits the superior properties (i.e., Dk=10.2, Df=0.001 9, and α= -405×10-6 ℃-1). This indicates that the as-prepared ceramic filler has the promising characteristics for potential applications of 5G high-frequency, high-dielectric PCBs.