Suffusion is a type of internal erosion in soil that fine particles detach from the matrix and get transported in pores with the seepage flow. The internal erosion rate is a key factor influencing the development and resulting consequences of the erosion process. In this study, a new method to evaluate the internal erosion rate is proposed, introducing the concept of the representative elementary volume of soil gradation in the model. The maximum particle size under erosion is related to the seepage velocity. The remaining mass of particles is described by a decay function for any given particle size. The eroded particles are assumed to get transported with the seepage water without dispersion. The effects of key parameters in the new method are analyzed using a finite element model, and a method is introduced to determine the parameters based on test results. Unlike existing methods, the proposed method considers the influence of soil gradation. The proposed method can accurately predict the development of internal erosion under different hydraulic conditions.