To address the stringent requirements of 5G/6G high-frequency and high-speed communication technologies for printed circuit board (PCB) substrates, a polytetrafluoroethylene (PTFE)-filled modified bismaleimide-triazine (BT) resin/glass fiber (GF) composite was designed and fabricated in this study. The glass fiber fabric was surface-modified with silane coupling agent KH570 to enhance interfacial bonding, and low-dielectric PTFE filler was introduced into the glass fiber cloth pores via high-temperature and high-pressure lamination process. The influence mechanism of PTFE content on the comprehensive properties of the composite was systematically investigated. Results demonstrate that when the PTFE mass fraction is 5%, the composite achieves the optimal comprehensive performance. The dielectric constant and dielectric loss are 4.3 and 0.006 at 1 MHz, and 3.4 and 0.003 at 10 GHz, respectively. The glass transition temperature increases to 250℃ (approximately 25℃ higher than that of the pure BT system), and the residual char yield at 800℃ reaches 76.63% (an increase of about 17.55%). The flexural strength is maintained at 574.7 MPa, and the water contact angle increases to 86.5°. Mechanism analysis reveals that the introduction of PTFE significantly improve the high-frequency dielectric properties of the composite by reducing the polarization degree, filling pores to decrease interfacial polarization losses, and restricting the orientation movement of polar groups. Simultaneously, the high bond energy of C-F bonds endows the composite with enhanced thermal stability. However, the high coefficient of thermal expansion (CTE) of PTFE raises the CTE of composite to approximately 45×10^-6℃^-1, which requires further optimization. This material system achieves significant improvement in dielectric properties while maintaining the high heat resistance and good processability of BT resin, providing a cost-competitive substrate solution for mid-to-high frequency applications such as 5G millimeter-wave antennas and radio frequency front ends.