Abstract: To investigate the dynamic characteristics of an extended electric toolholder and the formation mechanism of machining errors in deep cavity machining, an extended electric toolholder was selected as the research object. Hammer impact modal testing, unload variable-speed testing, and cutting process dynamic testing were conducted to identify the natural frequencies of the toolholder system and to evaluate the unloaded vibration displacement, cutting vibration response, elastic tool deflection, and rebound overcutting. The results indicate that the primary natural frequencies of the toolholder are 55, 250, 510, 620, 840, 1 150, 1 405, and 1 730 Hz, with the low-order mode near 55 Hz having a significant influence on dynamic stability. Under unload rotation, the radial displacement at the tool tip remains stable; the horizontal displacement decreases from 72.5 μm to 60 μm at 3 000 r/min, while the vertical displacement remains within 52-56.5 μm. The cutting process considerably amplifies the dynamic response. The elastic tool deflections under horizontal and vertical cutting are 239-248 μm and 242-275 μm, respectively, with rebound overcutting of 50-82 μm. The results demonstrate that, under the present deep cavity machining conditions, cutting-load-induced elastic deflection is the dominant factor affecting machining accuracy. Therefore, improving the stiffness of the toolholder front end and supporting connections, together with optimizing cutting parameters to reduce instantaneous cutting forces, should be prioritized.