With the analysis of experiment and theory on GaN HEMT devices under DC sweep， an improved model for kink effect based on advanced SPICE model for high electron mobility transistors （ASM-HEMT） is proposed， considering the relationship between the drain/gate-source voltage and kink effect. The improved model can not only accurately describe the trend of the drain-source current with the current collapse and kink effect， but also precisely fit different values of drain-source voltages at which the kink effect occurs under different gate-source voltages. Furthermore， it well characterizes the DC characteristics of GaN devices in the full operating range， with the fitting error less than 3%. To further verify the accuracy and convergence of the improved model， a load-pull system is built in ADS. The simulated result shows that although both the original ASM-HEMT and the improved model predict the output power for the maximum power matching of GaN devices well， the improved model predicts the power-added efficiency for the maximum efficiency matching more accurately， with 4% improved.