The calculation of bed load sediment transport rate is of great significance to the theoretical development of river dynamics and the practical application of water conservancy projects. In order to further improve the load transfer theory, this paper introduces the concepts of exposure height and exposure angle, and calculates the distribution law of the two through random distribution experiments. The analysis results show that both of them obey the normal distribution. Based on the randomness of water flow conditions and the distribution law of sediment position, the method of combining probability theory and mechanical analysis is used to derive the probability formula of entrainment probability. Using the modified basic model of Einstein's bed load sediment transport rate for reference, a new bed load sediment transport rate formula is established. The verification of measured data and the comparison with the classic bed load sediment transport rate formula and the bed load sediment transport rate formula based on the same theory show that the formula in this paper has a clearer physical concept and higher calculation accuracy.