Due to the advantages of high power density, high efficiency, and great potential in extreme temperature environments, the GaN HEMT device is widely used in circuit systems with 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 properties of the GaN HEMT device under different ambient temperatures, which is also crucial to for the accurate circuit design. With the analysis of experiment and theory on the GaN HEMT under 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 impact of the ambient temperature on electrical properties of the GaN HEMT, such as the threshold voltage, the knee voltage, the saturated current and so on, the model establishes temperature-dependent functions for key parameters in the formula of the drain-source current. Realized by Verilog-A and simulated on the ICCAP software, the improved model accurately reflects the changing trend of the electrical performance on the GaN HEMT under different ambient temperatures. To further verify the model in the paper, the on-wafer measurement under different temperatures including -55℃, -25℃, 25℃ and 75℃ is carried out for GaN HEMTs with different sizes, which are developed by Nanjing Electronic Devices Institute. Compared with the measured data, the proposed model simulates the output characteristics and the transfer characteristics accurately over the ambient temperature range of -55℃~75℃, with the relative fitting error less than 5%. The result shows that the improved model is of guiding significance for the analysis of the DC performance and high reliability design of circuits under different temperatures.