In order to explore the influence of hydraulic gradient on the seepage erosion in soil, fused silica sand was used as the transparent soil material, and the pore solution was configured with No.15 white oil and n-dodecane in a certain proportion.Based on the seepage erosion system developed by PIV-PLIF(particle image velocimetry and planar laser induced fluorescence), the influence of hydraulic gradient on the flow velocity distribution of pore solutions and the movement of fine particles in seepage erosion damage was explored. The results show that with the increase of the hydraulic gradient, the flow velocity of tracer particles also increases and the irregular motion phenomenon intensifies, and the flow velocity tends to be stable until the sample fails. Under the same hydraulic gradient, the macroscopic flow velocity is slightly larger than the microscopic flow velocity, and the gap between the two is also narrowing with the increase of the hydraulic gradient. When the hydraulic gradient reaches the critical hydraulic gradient, the fine particles in the particle skeleton begin to lose, and the permeability coefficient also begins to change.If the water head continues to increase, a large number of fine particles will be lost, resulting in the continuous development and expansion of the seepage channel, and finally the critical hydraulic gradient of failure will be reached.