<p>Dielectric substrate material with a low dielectric constant and dielectric loss is essential for high-speed microwave devices, offering significant potential in information processing and intelligent driving assistance applications. In this study, a polyphenylene oxide (PPO)-based covalent cross-linking network system was designed as a potential substrate material for high-speed microwave devices. Utilizing PPO with low dielectric constant and excellent thermal stability as matrix resin, thermoplastic elastomer styrene–butadiene–styrene copolymer (SBS) as toughening agent, and small molecule triallyl isocyanurate (TAIC) as active crosslinker enhanced the interfacial compatibility of the modified PPO (mPPO)-based composites system. The curing behavior, dielectric properties and thermal stability of cured mPPO systems were studied, and the impact of the PPO to TAIC ratio on the structure and performance of the cured mPPO systems were further discussed. When the mass fraction of PPO/SBS/TAIC components in the mPPO system is 40/30/30, the sample exhibits a low dielectric constant (<i>D</i>_<i>k</i> = 2.49@10&#xa0;GHz), moisture absorption (0.37%) and excellent thermal stability. In addition, it also maintains good bending strength and solvent resistance, indicating significant potential for applications in the field of electronic appliances. This work provides new insights and ideas for the preparation of PPO-based composite substrate.</p> Graphical Abstract <p></p>