Due to the unique properties of calcareous sand in reclamation projects, soil foundations consisting of calcareous sand-filled land are often exposed to reversible tidal waves. This may induce continuous long-term internal erosion of the soil that differs from monotonic seepage conditions, which has not yet been studied from a microscopic perspective. In this study, a selected particle size distribution (PSD) based on the breakage experiments of calcareous sand is adopted. Then, a series of reversed seepage flows is generated using CFD-DEM, taking into account both cyclic frequency and hydraulic gradient. Numerical results examine the overall suffused fine mass, the migration and suffusion degree across the divided layers along seepage, and the microscopic origins of continuous suffusion under reversed hydraulic conditions. Furthermore, the micro-mechanical and energetic evolution of soil skeletons is analyzed. The findings reveal that hydraulic gradient is the primary factor influencing suffusion outcomes at the initial stage (PEM increases from roughly 21% to 24%), while the cyclic frequency determines the long-term result (PEM increases from roughly 30% to 37%). From a microscopic point, the elimination of clogging, combined with energetic instability and force chain network rearrangement, is identified as the origin of continuous suffusion under reversed hydraulic loading.