The characteristic X-ray of a target is of considerable significance in industrial applications and medical diagnosis and treatment, and its intensity is closely related to the incident electron energy. At a high energy, it is not easy to determine the relation between characteristic X-rays and the incident electron energy through measurements, but the Monte Carlo method has a wide energy calculation range. In this study, the X-ray energy spectra of six target materials (Cu, Mo, Rh, Ag, W, and Pt) were simulated at various incident electron energies (<3 MeV) using the Monte Carlo code MCNP5 and the relation curve between the characteristic X-ray intensity of each of the target materials, and the incident electron energy was obtained through a simulation. A Si-PIN detector was used to measure the low-energy output energy spectra of two X-ray tubes (Ag and W targets). The relation curve between the X-ray tube excitation voltage and the characteristic X-ray intensity was obtained by fitting the measured data to a linear function. The simulation fitting curve and measurement fitting curve agreed well in the low-energy range. Comparisons of the calculated and measured values revealed that most of the deviations for the Ag target were less than 5%, and those for the W target were less than 6%.