Due to the comprehensive performance advantages, GaN-based power devices are more suitable for the future development needs of RF power amplifier modules in the space equipment such as satellite electronic systems．Therefore, the degradation of electrical characteristics and damage mechanism of the enhancement-mode Cascode structure GaN HEMT devices were studied by irradiation experiments with 5 MeV, 60 MeV and 300 MeV protons at the irradiation dose of 2×10¹²～1×10¹⁴ cm^-2. The experimental results show that when the irradiation dose is 2×10¹² cm^-2, the threshold voltage of the Cascode structure GaN HEMT device is significantly reduced, the transconductance peak is negatively drifted and the peak transconductance is reduced, the saturated drain current is significantly increased, and the gate leakage current has no significant change. When the irradiation dose reaches 1×10¹³ cm^-2, the degradation of electrical properties is inhibited and tends to saturate. It is concluded that the cascaded silicon MOSFET in the Cascode structure GaN HEMT is the internal cause of threshold voltage negative drift and drain current increase after proton irradiation. Combined with low-frequency noise test analysis, it is found that the higher the proton irradiation dose, the larger the noise power spectral density of the device, indicating that the more defects introduced by irradiation, the more serious the irradiation damage. Compared with the results of 60 MeV and 300 MeV proton irradiation, the degradation of electrical characteristics of the device after 5 MeV proton irradiation is the most serious. SRIM simulation results show that the lower the proton irradiation energy, the greater the number of vacancies (gallium vacancy is dominated), and the more significant the degradation of electrical characteristics of the device.