In this paper, a soil particle starting model based on the theory of sediment motion mechanics is established. Based on the relative exposure, the formula for calculating the critical velocity of internal erosion caused by soil seepage is derived. Comparing with the critical flow rate measured in the test, it is found that the test value is consistent with the calculated value. The difference between the calculated value of the critical flow rate and the experimental value and the relationship between the relative exposure and the critical flow rate are discussed. The accumulation of particles during the seepage process will cause the particle transport channel to block, and the critical flow rate test value is greater than the calculated value. The critical flow rate is positively correlated with the relative exposure of soil particles. The relative exposure degree has different influence on the critical flow rate in different seepage directions. The relative exposure has a great influence on horizontal seepage. For the vertical seepage, the relative exposure has little effect on the critical flow velocity.