<p>Conventional drilling (CD) of CFRP/Ti stacks often results in unsatisfactory surface quality, presenting a critical obstacle for the aviation industry. While the concept of applying high-frequency vibration along the axial direction of the drill bit to improve surface quality was introduced years ago, longitudinal-torsional composite ultrasonic vibration assisted drilling (LTUAD) has not been adequately studied. This study systematically explores the surface quality in LTUAD of CFRP/Ti stacks integrating theoretical analysis and experimental research. On the basis of the kinematic principle and cutting trajectory, the separation cutting characteristics and complete geometric chip breaking characteristics are deeply studied, and the conditions that meet the separation cutting and complete geometric chip breaking characteristics are derived theoretically. Machining experiments of LTUAD on CFRP/Ti stacks are carried out and regression models for surface roughness were established to predict the hole surface roughness under different machining parameters. Furthermore, the effect of machining parameters on the hole surface roughness are studied and obtain the best machining conditions for LTUAD of CFRP/Ti stacks. The research findings indicate that LTUAD demonstrates advantages over CD in improving the surface quality of holes. According to the influence rules of machining parameters on surface roughness, the surface roughness of the hole exhibits a significant negative correlation with the ultrasonic vibration amplitude, while showing a positive correlation with the spindle speed and feed rate. Within the processing parameters used in our experiments, the optimal parameters for minimum hole surface roughness of LTUAD of CFRP/Ti stacks were obtained as <i>A</i> = 9.8&#xa0;μm, <i>n</i> = 300 r/min and <i>f</i>_<i>z</i>=0.005&#xa0;mm/r.</p>