The effects of strain relaxation of AlGaN barrier layer on the conduction band profile, electron concentration and two-dimensional gas (2DEG) sheet charge density in a high Al-content AlGaN/GaN high electron mobility transistor (HEMT) are calculated by self-consistently solving Poisson's and Schr(o)dinger's equations. The effect of strain relaxation on dc I-V characteristics of AlxGa1-xN/GaN HEMT is obtained by developing a nonlinear charge-control model that describes the accurate relation of 2DEG sheet charge density and gate voltage. The model predicts a highest 2DEG sheet charge density of 2.42×1013 cm-2 and maximum saturation current of 2482.8 mA/mm at a gate bias of 2 V for 0.7 μm Al0.50Ga0.50N/GaN HEMT with strain relaxation r =0 and 1.49×1013 cm-2 and 1149.7 mA/mm with strain relaxation r =1. The comparison between simulations and physical measurements shows a good agreement. Results show that the effect of strain relaxation must be considered when analyzing the characteristics of high Al-content AlGaN/GaN HEMT theoretically, and the performance of the devices is improved by decreasing the strain relaxation of AlGaN barrier layer.