The analysis of the frequency dispersion characteristics of the gate-drain capacitance of GaN HEMT indicates that the gate fringe capacitance is responsible for the dispersion difference between the gate-drain capacitance and circle Schottky diode. By fitting the relationship between the additional capacitance of trap and frequency, we discover that the additional capacitance of trap can meet single energy level model only under small gate bias, and meet both single and consecutive energy level model under strong reverse gate bias. The gate fringe capacitance dispersion appears after SiN passivation. It suggests that the trap observed by fringe capacitance is introduced by passivation, which lies in the surface of the ungated region between source and drain. Finally, the low frequency noise technology is used to validate the feasibility of abstracting trap parameter by the gate fringe capacitance. The time constant of single energy level trap obtained by low frequency noise technology is consistent with the result obtained by the gate fringe capacitance under strong reverse gate bias.