A new type of RF small-signal equivalent circuit model and an analytical modeling method based on rational function fitting were proposed for 3 nm gate-all-around device. At first, the bias-independent extrinsic gate, source and drain resistance, gate-to-source/gate-to-drain capacitance, and substrate capacitance and resistance were extracted. Then, the bias-dependent intrinsic parasitic parameters were extracted under different bias of the device. The 3D TCAD and Matlab were used to simulate the device and get parasitic parameters by fitting, and the parameter results were put in ADS for verification. The results showed that the maximum error of S parameter between TCAD simulation and equivalent circuit simulation was less than 269% at 10 MHz~300 GHz frequency range. The accuracy of the established model and modeling method were verified. The research results had a reference value for RF IC design.