The macro mechanical properties of slope are determined by the meso parameters of soil particles and their motion. Although the stress and deformation characteristics of slope at the macro level can basically be obtained by the finite element method based on continuum model, it is difficult to reveal the deformation and instability mechanism of slope in the micro scale, and there are obvious limitations. The three-dimensional DEM-CFD model of fluid solid interaction of coal measure soil slope was established by coupling DEM and CFD. The meso mechanism of coal measures soil slope failure under rainfall was analyzed. The results show that the failure mode of coal measure soil slope simulated by DEM-CFD is mainly of rain erosion, and the slope sliding surface is predicted to be of approximate straight-line section, which is very close to the range of rain erosion of slope in outdoor model test. This shows that the numerical method is suitable to analyze the stability of coal measure soil slope. Micro parameters such as force chain, coordination number and porosity of soil particles in slope will change during the rainfall. For example, the porosity of particles on the top of slope changed from 0.35 in initial state to 0.8 in unstable state. The change of these micro parameters is directly related to the macro mechanical performance of the slope soil. In this paper, the law of the failure evolution of the coal measure soil slope under the rainfall was explained through the analysis of the micro parameters change of the particles. The research results of this paper not only provides theoretical basis for the protection design and construction of the coal measure soil slope in this area, but also provides a new way of analyzing the macro mechanical laws in geotechnical engineering from the micro perspective.