To improve the surface roughness of parts in laser metal direct manufacturing (LMDM), the mathematical model was established to calculate the optimal overlapping ratio. The result shows that the dimensions of parts have no influence on the optimal overlapping ratio, namely, the desired surface roughness always can be obtained with the overlapping ratio of 33.3%. In order to verify the theoretical model, 12 samples were prepared at different overlapping ratio, and their surface roughness was measured under the microscope. The measurement results fully demonstrate the validity of the theoretical model and show that the surface roughness is relatively good at the overlapping ratio in 30~45%. Therefore, in the actual LMDM process, the overlapping ratio can be set flexibly within the range according to the specific processing conditions.