A complete model for studying the effect of radiation induced strain relaxation on electric performance of AlGaN/GaN HEMT was built utilizing the degree of strain relaxation related to radiation dose and energy as a basic variable. The effects of radiation induced strain relaxation on carrier density, threshold voltage and I-V characteristic were systematically investigated. The calculation results indicated that decrease in polarization induced charges as a result of strain relaxation causes decrease in carrier density, increase in threshold volgate and degradation in saturation drain current according to a linear rule. All results showed that the effect of radiation induced strain relaxation on HEMTs with high Al content appears more prominent. The calculation results also imply that strain relaxation is one of the important radiation damage mechanisms, the sensitivity of Al content to radiation induced strain relaxation degree need to be considered when improving the figure of merit of HEMTs by virtue of enhancement in Al content.