The work aims to predict the microstructure of the machining surface by longitudinal-torsional compound ultrasonic end milling, to optimize the machining parameters. Kinematics analysis was conducted to the longitudinal-torsional compound ultrasonic end milling and the three-dimensional motion trajectory equation was established based on this. The tool trajectory was simulated and the machining characteristics of the tool were studied. By discretization of cutting edge and workpiece, a theoretical model of surface microstructure under longitudinal-torsional compound ultrasonic end milling was established and 3D surface simulation was carried out by MATLAB. Ultrasonic vibration cutting experiment was carried out to TC4 titanium alloy. Theoretical simulation and cutting test results showed that the surface microstructure characteristics caused by vibration became more obvious with the increase of amplitude in longitudinal-torsional ultrasonic compound end milling. The pit effect of machined surface microstructure was weakened with the increase of ratio of torsional longitudinal amplitude. When At/Al=0.55, the machined surface presented strip-shaped microstructure. Frequency and speed affected the density of surface microstructure units. The characteristics of microstructure of machined surface are related to machining parameters such as frequency, amplitude, ratio of torsional and longitudinal amplitude, and cutting speed. The variation trend of the machined surface obtained from the milling experiment is consistent with the theoretical model of the surface.