Based on the nuclear magnetic resonance technology, indoor seepage tests on tailings sand with different fine particle contents and relative densities were carried out to study the coupling mechanism of fine particle content and density on the seepage failure from a mesoscopic perspective. Subsequently, a method for calculating the contribution of different pores to permeability based on T2 spectra was proposed. The results show that the T2 spectra of tailings sand under seepage action present a three-peak structure. The connectivity of micro-pores and medium-pores is greater than that of large pores. The porosity of large pores keeps increasing, while that of micro-pores and medium-pores changes unstably. The increase of fine particle content and density has an adverse effect on the pore development. When seepage failure occurs, the micro-pores and medium-pores decrease significantly, while the large pores increase sharply. With the increase of fine particle content, the growth rate of permeability decreases, and there is a threshold of fine particle content, which decreases with the increase of density. Micro-pores and medium-pores contribute little to permeability, while large pores contribute more than 98%. Increasing the hydraulic gradient enables pores with larger diameters to participate in the seepage process. The research results can provide a theoretical reference for the safe operation of fine particle tailings dams.