<p>This study investigates the variation of cutting forces in longitudinal–torsional ultrasonic-assisted milling (LTUAM), with particular emphasis on the influence of toolpath curvature. A mechanistic milling force model integrating toolpath geometry and ultrasonic vibration parameters is established to predict the total cutting force through elemental force integration, while considering the instantaneous tool rotation angle and uncut chip thickness (UCT). The model is validated by comparative experiments under linear and circular toolpaths. The results show that, under a linear toolpath, the relative variation in cutting force at different amplitudes remains below 10%. In contrast, under a circular toolpath, when the toolpath rotates through 180°, the cutting forces in the X and Y directions exhibit approximately trigonometric variation, indicating that toolpath curvature dominates the cutting force characteristics. Ultrasonic vibration affects the cutting force by modulating the instantaneous displacement, rotation angle, and UCT of the tool. The findings highlight the critical role of toolpath geometry in ultrasonic-assisted milling and provide theoretical guidance for cutting force control and toolpath optimization in complex trajectory machining.</p>