Abstract: As one of the most important parameters reflecting drilling process and state, axial force has a direct impact on the quality of fiber reinforced plastic drilling. In order to analyze the variation regulations of axial force with process parameters in rotary ultrasonic trepanning drilling, the mathematical model of drilling force was established based on the kinematic characteristics of single abrasive, and the drilling experiment on glass fiber reinforced plastic was made. The results show that the axial force increases with the increase of spindle speed or decreases with the decrease of feed rate, and the variation trend is consistent with mathematical prediction model. Compared with conventional drilling, in rotary ultrasonic drilling process, the axial force and exit delamination factor decrease by 6%~25% and 8%~15% ,and the hole quality has effectively improve.