Aiming at the problem that the optimized processing parameters of LMD are extremely complicated to obtain, an effective method is proposed by constructing a dimensionless processing diagram in this paper. A group of dimensionless processing parameters applicable to LMD has been defined, and a dimensionless processing diagram has been constructed on the basis of parameter data available in the literature. The practicability of the dimensionless processing diagram has been proved experimentally for the LMD of Ti6Al4V. The optical micrographs show that the prior-β columnar grain morphologies of the as-deposited samples are a function of E0*. A high value of E0* leads to a relatively low cooling rate and coarse columnar grain. The cooling rate of the melt pool dictates the grain size formed in a deposited layer with a lower cooling rate, resulting in a coarse microstructure. By the experimental design of orthogonal array and ANOVA, the significance of processing parameters related to the microhardness of the LMD Ti6Al4V is q>v>h. The lack of fusion voids and gas pores is strongly affected by E0*, and these defects, in turn, affect the mechanical property. The ductility of the as-built samples is compromised at a lower value of E0*; however, owing to irregular lack of fusion voids, despite having comparable tensile strength with those samples at a higher value of E0*, the yield strength, ultimate strength, and elongation reach a maximum value of 890 MPa, 963 MPa, and 13.4%, respectively, at E0*=3.74, which exceed the forgings standard and is close to those fabricated by additive manufacturing reported in the literature. High-performance parts are obtained because of the dimensionless processing diagram constructed in this paper, which effectively narrows the processing window.