The dynamic drilling force in the ultrasonic assisted micro drilling (UAMD) process has an important influence on the tool wear and micropore quality. The dynamic drilling force model was established to predict and control the drilling force in the UAMD process, so as to improve the tool life and the micropore quality.A method for establishing thrust force dynamic model based on transfer function identification was proposed.In UAMD, the time response parameters of thrust force were obtained by using the drilling parameters under variable feed rate.The drilling system was modeled by using the system identification method, and the original transfer function model of the thrust force was obtained. Based on the results of preliminary verification and the frequency domain analysis of the transfer function, the poles and gains of the model were corrected by using a polynomial fitting method.The optimized mathematical model was transformed into the state space, and the dynamic response model of the thrust force of UAMD was obtained.The experimental results show that the optimized dynamic model is in good agreement with the time domain response of the experimental thrust force.