To simulate leaf area index (LAI) accurately is the key for the prediction of crop growth and yield in a crop growth model. Based on the analysis of the dynamic changes in the LAI of high-yielding barley cultivars in Wuhan and Yangzhou, a simulation model of barley LAI was established, in which, the LAI was the function of expansion coefficient of LAI for cultivar genetic property, climatic factors such as daily air temperature difference, sunshine hours, and accumulation of photosynthetic available radiation after sowing (sigma PAR), and limitation indices of water and nutrients. It was indicated that the maximum LAI and optimal LAI at the stages of booting and heading were not the same conception, but differed significantly. The model was tested by the field experiments with different barley cultivars under different sowing dates and nitrogen application rates in Yangzhou, Nanjing, and Kunming. The results showed that this model gave the good predictions of LAI at different development stages, with the RMSE values ranged in 0.742 and 2.865, and averaged 1.348. The simulated and observed LAI values were significantly positively correlated, and the correlation coefficient from y = x regression analysis was between 0.511 and 0.954.