With the analysis of experiment and theory on GaN HEMT devices under DC sweep, an improved model for kink effect based on 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 current collapse and kink effect, but also precisely fit different values of drain-source voltages at which 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 can predict the output power of GaN devices well, the improved model predicts the efficiency more accurately, 4% higher than the original ASM-HEMT.