Soil suffusion induced by long-term seepage generally lead to deformation or damage of soil structure. The suffusion experiments of silty sand had been accomplished by the independently developed seepage instrument, to study the particle loss process and suffusion evolution characteristics of silty sand with different void ratios induced by seepage of different hydraulic gradients, and the influential mechanism of factors is understood on the sand suffusion, then a model is established to predict the increase of particle loss with seepage duration and hydraulic gradient. Furthermore, the development patterns are revealed of particles size distribution and void ratio during suffusion based on the particle grading test and three phases relationship. The results show that: firstly, fine and sandy particles are both washed away by the long-term seepage and some dominant seepage channels will form subsequently in the samples, and then the internal local collapse and the overall differential settlement will be induced by continuous suffusion, until the water pressure mainly dissipates along the dominant channel to achieve the suffusion stability. Secondly, the initial void ratio or compactness is the main internal factor affecting the degree of the seepage suffusion of the soil, which directly changes the hydraulic gradient threshold, loss rate, cumulative total mass of particle movement and settlement deformation of sandy samples. The increase of the hydraulic gradient will also accelerate the particles loss, channel formation and suffusion stability. Thirdly, the prediction model fits the experiment result well, which can reflect evolution characteristics above and the influence on particles loss process of initial void ratio as well as the hydraulic gradient. Finally, the void ratio of samples will increase and the uniformity of particle size will decrease during sand suffusion.