Polymeric materials with high heat resistance and low dielectric constants are desired for fast communication. To realize this purpose, two kinds of silicon-containing main chain type benzoxazines with and without an acetylene group, (ABA-Si-ala)main and (ABA-Si-a)main, were designed and synthesized from polyphenol oligomers, primary amines, and paraformaldehyde. Their chemical structures were characterized and verified by Fourier transform infrared (FTIR) spectra and nuclear magnetic resonance (NMR) spectra. Moreover, their curing behaviors and polymerization reactions were studied by differential scanning calorimetry (DSC) and FTIR spectra, and cross-linking structures were proposed. Furthermore, thermal properties of the two polybenzoxazines were analyzed by dynamic mechanical analysis (DMA) and thermogravimetric analysis (TGA). The results showed that poly­(ABA-Si-ala)main exhibited excellent heat resistance and thermal stability, and its T g and char yield at 800 °C under N2 reached as high as 308 °C and 48.5%, respectively, which were higher than those of poly­(ABA-Si-a)main. In particular, the existence of the siloxane structure endowed the two polybenzoxazines with good dielectric properties, and the dielectric constants of poly­(ABA-Si-ala)main and poly­(ABA-Si-a)main were as low as 2.78 and 2.67 at 1 GHz, respectively. Such low dielectric constants will be necessary and vital for signal transmission. Therefore, this work provides a new and effective strategy for designing new polymers with both excellent heat resistance and low dielectric constants.