A semi-analytical small signal parameter extraction method for high electron-mobility transistor(HEMT) under different bias conditions is presented. Based on test structure to determine the pad capacitance and parasitic inductances, the semi-analysis method is used to extract parasitic resistances and to improve the precision of the parasitic resistance in the small signal model. The agreement between the measured S-parameters and simulated ones is excellent over the frequency range up to 40GHz under multibias condition.