The development of lightweight composites with desirable thermo‐mechanical properties is progressively increasing. Phthalonitrile (PN) based composites have shown great potential in this regard. However, the basic thermal properties of PN composites required for engineering design are not yet fully understood. In this work, we investigated the thermal stability, thermal expansion behavior and thermal conductivity of PN composites reinforced with carbon fiber (CF) and high silicon fiberglass (HSF) via combined experimental studies and numerical simulations. The results indicated that CF/PN performs better in thermostability than HSF/PN at temperatures below 500°C. Moreover, the incorporation of CF and HSF lowered the coefficient of thermal expansion (CTE) and thermal insulation of PN. At room temperature, the in‐plane CTE of CF/PN and HSF/PN were 1.97E‐6 and 9.24E‐6°C−1, respectively, while the out‐plane thermal conductivities of CF/PN and HSF/PN were 0.65 and 0.34 W/(m K). It is worth noting that an excessively high fiber volume fraction would lead to poor thermal insulation and lightweight properties of the PN composites, while a low fiber volume fraction would result in poor stiffness and thermal dimensional stability. Determined via TOPSIS model, a fiber volume fraction range of 50%–60% was ideal for both composites with comprehensive optimal properties, respectively. Preparation and simulation of PN composites.