In this study, we design a solar spectrum synthesis method based on monochrome LEDs to solve the problem of spectral discontinuity associated with white LED. The proposed method ensures the fitting of the solar spectrum by constructing a monochromatic LED spectral model by the least squares algorithm considering the maximum correlation index R² as the optimization target. Based on the ideal peak-wavelength equal-interval LED fitting, the simulation experiment is solved to obtain an optimal fitting solution for a non-equal-interval monochrome LED. Thus, we obtain an optimum number (27) of monochrome LED combinations, and the maximum correlation index R² is 0.9576. Furthermore, the spectral fitting result of two groups of 17 monochromatic LED combinations obtained by decreasing experiment is analyzed. The mixed spectrum meets the Class-A-level standard requirements of AM1.5. Thus, the best fit of various LEDs is achieved with respect to the solar spectrum, and the difficulty of engineering implementation is reduced.