Abstract Salt pans are commonly found in coastal marshes and play a vital role in the marsh plant zonation. However, the correlation between these hypersaline zones and the marsh hydrological conditions have barely been characterized. This study numerically investigates the effects of evaporation rate, tidal amplitude, and marsh platform slope on salt pan formation, and found that salt pans can hardly grow in the intertidal zone due to regular tidal flushing, while tend to form in the lower supratidal zone, where evaporation is sustained. The accumulated salts create an upward salinity gradient that trigger downward unstable flow. The decreases of potential evaporation rate, tidal amplitude and/or marsh platform slope strengthen the hydraulic connection between the marsh surface and the underlying watertable, the key to sustaining evaporation, and therefore result in thickener and wider salt pans. These findings offer a deeper insight into the marsh eco-hydrology and guidance for their degradation prevention.