To determine the optimal stemming length for hard rock weakening in underground coal mines, this study investigates the motion law of the stemming structure. Using LS-DYNA, the movement of the stemming structure under both rigid piston compression and explosive blasting was simulated. The results reveal non-uniform compression within the borehole, with maximum density at the compression end and minimum at the free end, followed by rigid-body motion post-compression. Based on these characteristics, a one-dimensional axial motion mechanical model was established. The calculation of explosion gas pressure was systematically improved by considering its attenuation due to rock failure, stemming compression, and movement. A "time-segmented" solution method was applied to analyze the stemming motion process, and a corresponding algorithm was designed. Finally, the method was validated in a hard rock deep hole weakening project at a coal mine heading face.