In this study, we investigated the horizontal distribution of salinity gradient in a tidal channel by utilizing the Fluvial Acoustic Tomography System (FATS). Two pairs of FATS transducers were installed across the river, with the maximum horizontal distance between them exceeding 700 meters. Data were collected over three periods, encompassing both neap and spring tidal phases. The results demonstrate the effectiveness of FATS in continuously monitoring salinity gradients within the tidal channel. Notably, our findings reveal a significant increase in seaward axial salinity gradients at the monitoring location over the study period. Moreover, the longitudinal salinity gradient during spring tides was approximately twice as high as during neap tides.