This paper proposes a dimming calculation model for red/green/blue/warm-white (R/G/B/WW) light emitting diode (LED) based on four channels’ pulse width modulation (PWM). According to the characteristics of two-channel and three-channel PWM, the proposed model adopts Chebyshev method of black body locus to determine the relationship between chromaticity coordinate and correlated color temperature(CCT) of the synthesized light. When the color rendering performance is optimized, the functional relationships between chromaticity coordinate and duty cycles, CCT and duty cycles of the mixed light are established. And the experimental verification is carried out with R/B/G/WW LED. The experimental results show that R/G/B/WW LED module can realize a wide range of CCT tunable white light from 2900 K to 7600 K. When the luminous flux is set to 300 lm, the maximum relative error of CCT is 0.99%, and the maximum relative error of color rendering index (CRI) of the mixed light is 0.11%. When the luminous flux changes in the range of [150,800], the maximum relative error is 2.02%. The model can be applied to LED dimming of four channels. And its calculation method is simple, the dimming precision is high, and the hardware is easy to implement.