Aiming　at　the　improvement　of　typical　equivalent-circuit　model＇s　performance　on　precision,　service　band　and　compatibility　with　network　analysis　method,　from　ac　I-V　equations,　this　paper　calculated　SiGe　HBTs　two　port　network　parameter　model　described　by　scattering　parameters.　In　the　calculation,　the　influence　on　HBTs＇ac　characteristics　by　Si/SiGe　heterojunction　and　base　structure　were　emphatically　analyzed,　and　the　reject　of　physical　quantities　and　the　limit　to　frequency　were　avoided　as　much　as　possible　to　improve　the　model＇s　accuracy　and　frequency　range,　which　provided　a　wideband,　high-precision　quantitative　approach　for　the　work　of　the　device　design,　optimization　and　applications.