We present a tunable, general-purpose, light-emitting diode-based spectrally tunable light source that achieves high-precision fitting of chlorophyll b absorption spectra from 400 to 700 nm using 11 LEDs with different peak wavelengths. The target spectrum is divided into gentle and steep segments, and the optimal number of LEDs to be used at different wavelengths is determined by the segment fitting method. Then, the results were further optimized by fitting experiments, and finally the best combination consisting of 11 different peak wavelength LEDs was obtained. The results showed that the correlation coefficient R² reached 0.9664. The precise control of each LED current by means of the pulse width modulation signal output from the microcontroller, and the output spectrum of the system was measured using a spectrometer. The results show that the correlation coefficient R² reaches 0.9315.