This paper presents a dimming method that based on three channels′ pulse- width modulation (PWM) and light- mixing technology. A quantitative calculation model is established among chromaticity coordinate, correlated color temperature and three channels′ duty cycles. The functional relationship between chromaticity coordinate and maximum luminous flux is also setuped. This method has been verified favorably by light mixing experiments with tricolor LED, which shows that the deviation of chromaticity is less than 2.5% between the theoretical value and the measured value. In addition, the color temperature change of sunlight has been simulated by this quantitative calculation model, and the deviation of CCT is less than 50 K. This model with the advantage of high precision can be applied to various types of LED dimming color and has a beneficial value for practicality.