Block in matrix (BIM) soil is composed of block stone, gravel, sand, clay and other solid particles. The model for seepage erosion in BIM soil is assumed as the form of Sterpi Formula, and the content of block stone as one of the most important parameters of BIM soil needs to exist in the model. However, due to the complexity of grain sizes in BIM soil and the limitation of traditional experimental setup in the laboratory, it is hard to conduct experiments directly to study seepage erosion in BIM. Therefore, a new method is proposed to obtain these parameters by means of numerical simulation on BIM soil and experiments of seepage erosion on gravel soil matrix without block stones. BIM soil from a landslide in Yunnan Province is selected as an example. Firstly, an experimental setup is designed to perform a series of seepage erosion tests on gravel soil matrix without block stones to obtain its seepage parameters. Then, based on the governing equations of particle migration, coupling seepage and porosity, numerical simulation is carried out to characterize the process of seepage erosion in BIM soil under the assumption of block stones with poor permeability. Numerical results show that when hydraulic gradient and time are consistent, the average initial velocity of BIM soil is inversely proportional to the content of block stone, but the density of liquefied particles is proportion to the content of block stone. Finally, a formula is presented for seepage erosion in BIM soil with the content of block stone as one of parameters.