In this paper,first,the Bleached Chemical Thermal-Mechanical Pulps（BCTMPs） of poplar,fir and maple were irradiated with UV light to accelerate the yellowing.Then,the variations of brightness（R） and yellowing number（PC number） of these three kinds of BCTMPs during the light-induced yellowing were investigated.Finally,a model to describe the variations of brightness and PC number with the irradiation time is established based on regression analysis.Analytical results show that there is a linear relationship between PC number and b^＊ and a sub-linear relationship between lg t and R as well as PC number.Experimental results indicate that the proposed model is generally applicable.