Phytoremediation biowaste without proper disposal might release potentially toxic metals (PTMs) back into the environment and pose a serious threat to environmental safety and human health. A composite geopolymer based on phytoremediation biowastes (PBWs) was used to effectively solidify/stabilize toxic heavy metals and served as an insulation material in the present study. For the solidification/stabilization of PTMs, the optimum proportion of biowastes in the geopolymer insulation material was 3%, at which the compressive strength of the geopolymer reached a maximum of 32.6 MPa and the thermal conductivity of the geopolymer was 0.11 W/(m·K), meeting the requirements of insulation materials. The leaching results indicated that the solidification efficiency of PTMs in the geopolymer materials was over 98%. The leaching concentration of all samples was within the standard limits and decreased with the extension of curing time. The XRD and FT-IR spectra of the biowaste-geopolymer confirmed the effective solidification/stabilization of PTMs in the geopolymer composite, where Zn was immobilized as carbonate and silicate. The geopolymer technique can stabilize the PTMs and prepare insulation materials simultaneously. Hence, this research provided a helpful insight and theoretical support for the post-treatment and the reutilization of plant waste accumulated with heavy metals.