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The small-signal S parameters of the fabricated double-finger gate AlGaN/GaN high electron mobility transistors (HEMTs) were measured at various direct current quiescent operating points (DCQOPs). Under active bias conditions, small-signal equivalent circuit (SSEC) parameters such as R s and R d , and intrinsic parameters were extracted. Utilizing f T and the SSEC parameters, the effective electron velocity (νe-eff) and intrinsic electron velocity (νe-int) corresponding to each gate bias (V GS ) were obtained. Under active bias conditions, the influence mechanism of V GS on νe-eff was systematically studied, and an expression was established that correlates νe-eff, νe-int, and bias-dependent parasitic resistances. Through the analysis of the main scattering mechanisms in AlGaN/GaN HEMTs, it has been discovered that the impact of V GS on νe-eff should be comprehensively analyzed from the aspects of νe-int and parasitic resistances. On the one hand, changes in V GS influence the intensity of polar optical phonon (POP) scattering and polarization Coulomb field (PCF) scattering, which lead to changes in νe-int dependent on V GS . The trend of νe-int with changes in V GS plays a dominant role in determining the trend of νe-eff with changes in V GS . On the other hand, both POP scattering and PCF scattering affect νe-eff through their impact on parasitic resistance. Since there is a difference in the additional scattering potential corresponding to the additional polarization charges ( APC ) between the gate-source/drain regions and the region under the gate, the mutual effects of PCF scattering on the under-gate electron system and the gate-source/drain electron system should be considered when adjusting the PCF scattering intensity through device structure optimization to improve linearity. This study contributes to a new understanding of the electron transport mechanisms in AlGaN/GaN HEMTs and provides a novel theoretical basis for improving device performance.