On the basis of analyzing the temporal and spatial distribution of leaf area in cotton canopy, a dynamic simulation model of leaf area vertical distribution and leaf area index in cotton canopy were developed by using the cumulative relative thermal effectiveness. The model was validated by the experimental data of different sowing dates and different years. The coefficient of determination (R2) based on the 1∶1 line and the root mean square error between simulated and measured L(z) were 0.969 and 0.257 respectively. The coefficient of determination (R2) based on the 1∶1 line and the root mean square error between simulated and measured LAI were 0.936 and 0.305 respectively. It proved that simulation results of the model came very close to the actual values. The simulation results could be coupled into the radiation transfer model and photosynthesis model of cotton.