Aiming at the complexity of optimizing parameter-extraction, a new parameter-extraction method for parasitic capacitance of GaAs HEMT was proposed. When parasitic capacitance was extracted, an appropriate optimization range was set to carry out optimization reference. The three times parameter optimization was adopted to ensure the optimization accuracy and model accuracy, avoid the cycle optimization, and improve the efficiency of parameter-extraction and parameter-optimizing. This method was independent of the device structure, which reduced the error caused by the assumption of the device structure. The model parameters of 17-element small signal equivalent circuit were extracted, and the reliability of the method was verified. The results showed that the S parameter was fitting well with the measured one, and the highest fitting frequency could reach 30 GHz.