This work presents a theoretical study of the diafiltration and concentration processes on xylo-oligosaccharides (XOs) syrup using nanofiltration (NF). In our previous study of the separation performance of the NF process on XOs syrup, the models of flux (Jv) and observed retention (Robs) of an NF membrane have been established. In the present study, these models were employed to construct the balance equations of syrup volume and XOs concentration in a feed tank. In experiments, the constant-volume continuous diafiltration (CVCD) and the discontinuous diafiltration processes were implemented. In CVCD simulation, the flux curve predicted by the model agreed well with the experimental data. In discontinuous diafiltration simulation, the calculated total content of XOs was 94.5%, which was about 1.5% less than the experimental result (96.11%) given by discontinuous diafiltration operation on a XOs syrup with initial XOs content 83.5%.