Abstract: The observed 1998-flood peak stage is 1.78 m higher than that of 1954-flood at Luoshan hydrographic Station of the Changjiang River middle reach. The 4 main reasons, summarized in this paper, are: ①the difference of flood characteristics；②the effect of 1954-flood detention and diversion；③the changes of the Dongting Lake inflow and outflow； and ④the fluvial processes of the Changjiang River mainstream channels. To quantitatively study the effects of the individual or combinative above-mentioned reasons, a 1-D flood routing hydrodynamic mathematical model has been developed, which can be applied to the Changjiang middle river channels. Based on 81.7-flood and 1998-flood calibrating calculations, 1954-flood restoring calculation and 1998-flood and 1954-flood routing calculations have been implemented by means of the model. The results and analysis show that: ①1954-flood detention and diversion enabled 1954-flood peak stage at Luoshan Hydrographic Station to decrease 0.83 m (i.e. 1998-flood peak stage would increase 0.83 m), ②due to the combinative effects of changes of the Dongting Lake inflow and outflow and fluvial processes of the Changjiang River mainstream channels, 1998-flood peak stage would raise 0.74-0.84 m, and ③because of fluvial processes of the Changjiang River mainstream channels, 1998-flood peak stage at Luoshan Hydrographic Station would increase 0.33-0.36 m.