Abstract: In response to the phenomenon of intensified permeability, deformation, and damage of ancient sea dike silty soil caused by tidal fluctuations or tidal surges generating negative pressure, a horizontal permeability deformation apparatus capable of simulating the influence of negative pressure was developed. Based on this apparatus, permeability deformation tests were conducted under different negative pressures (0, −3, −6, −9 kPa) and different dry densities of silty soil (1.40, 1.45, 1.50, 1.55 g/cm³). The results indicate that with increasing dry density, the critical hydraulic gradient increases, enhancing the resistance to permeation. However, negative pressure reduces the critical hydraulic gradient correspondingly and accelerates the permeation damage process of the silty soil. Through analysis, a comprehensive judgment method for the critical hydraulic gradient of silty soil is proposed, which mainly relies on the double logarithmic curve of hydraulic gradient and flow velocity determined by current specifications, supplemented by the distribution curve of permeation effluent particles. The experimental results can provide a reference for the design and evaluation of impermeable silty soil in strong tidal surge sea dikes.