In order to obtain smooth surface of thin-walled parts, single-trace cladding experiments are carried out for DZ125L superalloy by laser metal direct forming (LMDF) technology using different combinations of process parameters. Influence of the process parameters (such as laser power, scanning velocity, powder mass flow rate and Z-increment ) on height, width, aspect ratio and forming quality of single-trace cladding layer is studied systematically, by which the process parameters are optimized. In order to characterize geometrical features of single-trace cladding layer, a new method of three-dimensional (3D) characterization of energy density is proposed and the corresponding formulas are derived. From aspect ratio, dilution rate and energy density, the matching relationship between good cladding layer and its process parameters is clarified, which provides the foundations for further research into determining the process parameters according to design of cladding layer size. Experimental results show that the forming quality of thin-walled parts is obviously improved with smooth surface and no macro-defect by optimizing the process parameters.