To understand the non-equilibrium morphological adjustment of a river to environmental changes, it is essential to (i) identify accurately how previous water and sediment conditions have impacted the current morphological adjustment of the river to environmental changes and (ii) establish a corresponding simulation method for non-equilibrium conditions. Based on water-discharge and suspended sediment concentration (SSC) data and 82 cross-sectional data for the Huayuankou-Lijin reach of the Lower Yellow River for 1965-2015, the adjustment processes of the main-channel geometry (area, width, depth, and geomorphic coefficient) and their responses to changes in water discharge and SSC for different reaches are analyzed statistically. Then, with the water and sediment conditions as the main controlling factors, a delayed response model (DRM) of the main-channel geometry subjected to previous changes in water discharge and SSC is established using the multi-step analytical model. The results show that the main-channel area, width, and depth decreased initially, then increased, then decreased again, and finally increased again. They were correlated positively with the 4 a moving average discharge and negatively with the 4 a moving average SSC. The main-channel geomorphic coefficient for the Huayuankou-Sunkou reach exhibited a decreasing trend, whereas that for the Sunkou-Lijin reach decreased initially, then increased, then decreased again, and finally increased again. Except for the Huayuankou-Gaocun reach for 1965-1999, the coefficient was correlated negatively with the 4 a moving average discharge and positively with the 4 a moving average SSC. In applying the DRM to the response of the main-channel cross-sectional geometry to previous water and sediment conditions in the Lower Yellow River, the calculated values of the main-channel morphological parameters for all the sub-reaches agree well with the measured values. This indicates that the DRM can be used to simulate the response adjustment process of the main-channel cross-sectional geometry to variations in the water and sediment conditions. The results of the established model show that the adjustment of the main-channel cross-sectional geometry is affected by the current discharge and SSC (30%) and those of the previous seven years (70%), where the numbers in brackets are the respective weight factors. The established model offers insights into the mechanism whereby previous water and sediment conditions influence the current morphological adjustment of the river, and it provides an effective method for predicting the magnitude and trend of the main-channel geometry under different incoming water and sediment conditions.