With the development of communication technology, the gate length of HEMT devices becomes shorter and shorter, and the early speed-field empirical formula can no longer accurately describe this change as the gate length continues to decrease. Through the computer simulation of the existing velocity-field empirical formula, it is found that there is a certain error between it and the measured literature data, so an improved velocity-field empirical formula is proposed. Based on the linear charge control model, a new high-electron mobility transistor (HEMT) I.-V. model is analyzed by considering the channel length modulation effect. The simulation results show that the model has high accuracy.