<p>BNT (BaO–Nd₂O₃–TiO₂) system was modified via doping using sol–gel method, and BNST-Mn/PTFE microwave dielectric composite was prepared by mechanical blending. Scanning electron microscopy (SEM), thermal dilatometer, and split post-dielectric resonator (SPDR) were used to characterize the microstructure, dielectric properties, and thermal properties of the composite materials. The results indicate that as the amount of ceramic filler supplied grows, the agglomeration of fillers becomes more pronounced. At a filler content of 10 wt%, the dispersion of the filler in the matrix resin is relatively uniform. The dielectric constant and dielectric loss of the composites increased, and the bonding ability between the two phases diminished with the increase in filler content. When the mass fraction of the ceramic was increased to 70 wt%, the dielectric constant of the composite was 10.92 and the <i>Q·f</i> value was measured at 3584 GHz. At the same time, the coefficient of thermal expansion (CTE) of the composites reached 23 ppm/°C, which was a good compatibility with the copper foil. Lichtenecker equation, Maxwell–Wagner equation, Jayasundere–Smith equation, and effective medium theory equation (EMT) were used to predict the dielectric constant of the composites. In comparison, the dielectric constant values calculated using the EMT equation were in good agreement with the experimental values, especially at low filling volumes. Also, the simulation results of the thermal expansion coefficient of composite materials using the Lichtenecker equation and its improved form are in good agreement with the experimental values.</p>