In order to study the dynamic response of riverbed morphology of alluvial rivers to water and sediment conditions, the adjustment process of riverbed cross-sectional morphology at the Shashi segment of the upper Jingjiang Reach was investigated based on the delayed response principle of riverbed evolution. The multi-time scale rules of water and sediment series of the Shashi segment during the period from 1956 to 2011 were analyzed using the Morlet wavelet method, and a power-exponent relationship between the discharge, sediment load, and cross-sectional area was obtained with the moving average method. The results show that the adjustment of riverbed cross-sectional morphology of the Shashi segment is delayed with the change of water and sediment conditions and it is influenced by previous and present water and sediment conditions. The model for calculation of the cross-sectional area for water and sediment series in different time periods is established. The simulated results indicate that the simulated cross-sectional areas and actual values are in good agreement and their correlation coefficient R² is 0. 85, indicating that the established model can simulate the adjustment regularity of riverbed cross-sectional morphology of the Shashi segment of upper Jingjiang Reach.