Abstract: During the wire arc additive manufacturing process, an increase in the temperature of the deposited layer can disrupt the equilibrium of molten pool flow, leading to the emergence of the humping phenomenon. This issue can be alleviated by extending the interlayer waiting time, but it cannot be completely eradicated. In this paper, a regression model relating the surface roughness of the milled layer to the milling process parameters was established using the response surface methodology. This model elucidates the impact of the surface roughness of the milled layer on the flow characteristics of subsequent deposited layers. The findings indicate that a milled layer with lower surface roughness results in higher forming quality of subsequent deposited layers. When the surface roughness of the milled layer is 2.08 μm, the wetting angle of the deposited layer approaches 90°, and the forming flatness is optimal.