A thermosetting resin composition contains a primary resin formed from mixing a styrene-type polyphenylene ether resin thermally modified with styrene with an acrylic-type polyphenylene ether resin thermally modified with acrylic at a weight ratio ranging between 0.5 and 1.5, consequently having excellent heat resistance, flowability, and filling ability; and when cured, having a dielectric constant smaller than 3.0 and a dielectric dissipation factor smaller less than 0.0020 at the frequency of 1 GHz as well as a glass transition temperature higher than 210° C.; in application, the composition is suitable to impregnate reinforcement to form prepregs with excellent curability.