Due to the advantages of high power density, high efficiency, and great potential in extreme temperature environments, the GaN high electron mobility transistor (HEMT) device is widely used in circuit systems at high or low temperatures. However, its electrical performance is sensitive to the ambient temperature. Therefore, it is essential to build a model that can accurately characterize the electrical performance of GaN HEMTs at different ambient temperatures, which is also essential for precise circuit design. With the analysis of experiment and theory on the GaN HEMT at different ambient temperatures, an improved model for temperature effect on the DC characteristics of the GaN HEMT is proposed based on EEHEMT model. Considering the influence of the ambient temperature on electrical properties of the GaN HEMT, such as the threshold voltage, the knee voltage, and the saturated current, the model establishes a temperature-dependent function for key parameters in the formula of the drain-source current. Through Verilog-A implementation and simulation on the ICCAP software, the improved model accurately reflects the trend of the electrical performance changes of the GaN HEMT at different ambient temperatures. To further verify the model in this work, the on-wafer measurements at different temperatures including –55, –25, 25 and 75 ℃ are carried out for GaN HEMTs with different sizes, which are developed by Nanjing Electronic Devices Institute. Compared with the measured data, the output characteristics and the transfer characteristics simulated by the proposed model are accurate in an ambient temperature range of –55–75 ℃, with a relative fitting error less than 5%. The result shows that the improved model is of guiding significance in analyzing the direct current performance and high reliability design of circuits at different temperatures.