<p>Adjusting the wood flour content enables optimisation of wood–plastic composites (WPCs) for different applications. The effect of wood flour content on the machining performance of WPCs is considerable, yet its underlying mechanism remains unclear. In this study, orthogonal cutting experiments were conducted to evaluate the effects of wood flour content, cutting speed, and cutting depth on cutting force and surface characteristics. When the wood flour content increased from 30 to 50%, the higher composite strength resulted in greater cutting forces. However, at 70%, the weakened interfacial bonding led to a reduction in cutting force. Increasing wood content also decreased plasticity and increased brittleness, causing surface damage to shift from burr formation to more severe pit defects, thereby raising surface roughness. Increasing the cutting depth increased the cutting force (243–310%) and surface roughness (24–333%). Increasing the cutting speed also led to increases in cutting force (18–88%) and surface roughness (18–34%). Moreover, with higher wood content, cutting depth contributed more strongly to surface roughness variation, whereas cutting speed contributed more strongly to cutting force variation. These findings clarify the mechanisms of surface damage in WPCs with varying wood flour fractions and provide theoretical guidance for optimising their machining performance.</p>