To study the dynamics of flood propagation and discharge capacity in the downstream tidal river section of the Tanjiang River Basin under flood and tide overlapping conditions, a one-dimensional hydrodynamic model was constructed to quantify the safety threshold of flow discharge at key control node of the Tanjiang River Basin and flood discharge capacity during tidal cycles under various tidal levels, control water levels, and discharge durations, and reveal the hydrological patterns of the Tanjiang River Basin under combined influences of floods and tides through simulation and calculation in multiple scenarios. Then, a high-precision safe flood discharge threshold map was obtained by using the the random frest model. The results indicate that in the downstream tidal river section of the Tanjiang River Basin dominated by tidal dynamics, the water level in the main channel is primarily affected by downstream tidal level, with the flood-tide backwater effect being more pronounced in high tides, and the safe threshold of flood discharge is affected by both tidal level and discharge duration. The flood discharge capacity in ebb tide period is significantly higher than that in flood tide period, and the flood discharge capacity in ebb tide period generally exhibits a linear variation with the water level of control section.