To realize dynamic lighting of LED, a dimming method based on two channels’ Pulse Width Modulation (PWM) and light-mixing technology was presented. According to the analysis on geometrical, photometric, colorimetric and electrodynamic constraints, the determinacy and limitation of two channels’ PWM dimming for realizing Expected Photometric and Colorimetric Quantities (EPCQ) were demonstrated, and a quantitative model was established, which mapped the EPCQ into duty cycles. To examine the model, a light-mixing experiment with two kinds of white LED simulated variations of illuminance and correlation color temperature from dawn to midday. Mean deviations between theoretical values and measured values were obtained, which were 15 lx and 23 K respectively. Result shows that this method can effectively realize the light spectrum which has a specific requirement of EPCQ.