Fly ash is waste from thermal power plants that not only has a serious impact on soil and water but also accumulates in landfills and must be treated properly. In addition, plastic waste, especially polyethylene terephthalate bottles, deteriorates for a long time, so there is an urgent need to recycle them into valuable products. To solve the environmental pollution mentioned above, a feasible zero-waste method has been developed that converts 100% of fly ash into a lightweight composite aerogel reinforced with recycled polyethylene terephthalate fiber. In this study, raw fly ash is bound to the skeleton of the fibers by a non-toxic, biodegradable xanthan gum solution as a binder, which is then lyophilized, leaving a hollow porous structure. The obtained composite aerogels display extremely low density of 0.026–0.062 g/cm³, high porosity of 96.59–98.42%, low thermal conductivity of 34–39 mW/(m·K), flexibility with Young's modulus of 3.98–20.61 kPa, and noise reduction coefficient of 0.18–0.31. Adding fly ash to the fiber framework results in a lightweight composite aerogel with higher porosity, heat, and acoustic insulation, as well as impressive compressive modulus, compared to the fly ash-free aerogels. This study provides a safe, environmentally friendly, and cost-effective solution for recycling fly ash into high-value engineering materials.