The present disclosure relates to a duty cycle adjusting circuit of a lighting system and method thereof for sensing light of a first color and a second color emitted by a light emitting device of the first color and a light emitting device of the second color to generate corresponding intensity values. Then a first pulse width correcting value and a second pulse width correcting value can be calculated for adjusting a first duty cycle and a second duty cycle of a driving current to adjust the illuminating intensity of the light emitting devices of the first and the second colors. Through these adjusting steps, the light of the first and the second colors is within a predetermined range of color temperature. Therefore, light source for stable light in color temperature is provided.