To analyze the status of spawning habitat for the four major Chinese carps in the lower reaches of the Ganjiang River under varying hydrodynamic conditions, a physical habitat model for fish spawning habitat was established by coupling the two-dimensional hydrodynamic model and the habitat suitability model based on the fuzzy logic method. This model was used to simulate the spatial distribution of suitable spawning areas for the four major carps in the lower reaches of the Ganjiang River under different flow rates, to analyze the variation patterns of the weighted usable area (WUA) of spawning habitat, and to determine the appropriate ecological flow required for spawning. The results show that the overall suitability of the main branch was higher than that of the middle branch and the south branch when the flow rate was low. When the flow was high, the overall suitability of the main branch decreased obviously while the suitability of the middle branch and the south branch increased. The WUA of spawning habitat increased initially and then decreased with increasing flow rate. Compared with the low-flow condition, the WUA decreased faster under the high-flow condition. When the flow rate was 4 200 m³/s, the WUA was the largest, accounting for 22.97% of the study area. The suitable ecological flow range of spawning habitat for the four major Chinese carps was between 3 500 and 4 800 m³/s, and the optimum ecological flow was 4 200 m³/s.