For the difficulty to accurately regulate the color and luminous flux of light-emitting diode (LED) light source, and the regulation operation needs to be carried out in computers and other equipment in the production and application, a mathematical model representing the relationship between pulse width modulation (PWM) and LED illumination based on the Grassmann color law and the characteristics of PWM in regulating LEDs was proposed. The RGB LED dimming and color matching system was designed with STM32 microcontroller as the core. The monochrome, double-color, and trichrome mixed illumination experiments were performed on the system. The comparison between the measured value of the digital illuminometer and the theoretical value of the illumination mathematical model proves that the illumination error of the system in the range of 0~370 lx is less than or equal to 4%, and a total of 16 777 216 kinds of light colors can be synthesized. The control of the color and luminous flux of the light source can be accomplished directly on the system without connecting to other equipment, which can reduce the operation process and meet the color and illuminance requirements for industrial visual detection, crop fill lighting and cultural tourism.