Abstract: Exploring the transport characteristics of fine particles is of great engineering significance for clarifying the law of seepage erosion, as the content of fine particles will directly affect the resistance of soils to erodibility. Based on the self-developed PIV/PLIF seepage erosion test system, the effects of the initial content of fine particles on the flow velocity and the critical hydraulic gradient during the seepage erosion process are investigated. The results show that: (1) The process of seepage erosion can be basically divided into three phases, i.e., stable, localized destruction and overall destruction, according to the evolution law of flow velocity. This leads to the definition of the critical hydraulic gradients for localized and overall destruction, and the critical hydraulic gradients of the specimen increase with the increase of the initial content of fine particles. (2) When the initial content of fine particles is less than 20%, the specimen is in an underfilled state, exhibiting an overall destruction phenomenon, and the actual measured average flow velocity of the specimen accelerates with the increase of the hydraulic gradient after overall destruction. When the initial content of fine particles exceeds 30%, the specimen is essentially in a fully filled state, with an extremely high resistance to erodibility and the overall measured average flow speed of the specimen is relatively low. (3) During the phase of localized destruction, the rate of variation in the measured average flow speed of the specimen decreases significantly with the increase of the initial content of fine particles. It is known that the initial content of fine particles has a significant impact on the microscopic mechanism of seepage erosion for the specimen.