In this paper, an improved 16-element small-signal topology for InAlAs/InGaAs InP-based HEMTs has been proposed. The gate-source resistance (Rgs) is introduced into the topological structure to characterize the gate leakage current caused by the short gate channel spacing. The output conductance (gds) and drain delay factor (τds) are proposed to characterize the impact of drain voltage on channel current and also the phase change by drain-source capacitor (Cds), which can improve the fitting accuracy of S22. The parasitic elements are calculated through open and short dummy structures, and the intrinsic parameters are extracted by Y-parameters after de-embedding the external parasitic parameters. The ultimate values of parameters are determined by optimization procedure to gain the best fitting precision. The results show that the simulation values of S-parameters and frequency characteristics fit well with the measured values, and the introduction of Rgs and τds reduces the model error. The accurate and appropriate small signal model for InP-based HEMTs would be of great importance in the design of high-frequency circuits.