In response to the high-quality requirements for large-diameter deep-hole blasting, and the traditional stemming material being too long and prone to “blowout”, a new type of blasting hole stemming material was proposed using early strength cement mortar and a bag-like structure. Static mechanical tests and blockage model tests under impact loads were conducted on cement slurry with different admixture contents and ages, and the self-shrinkage performance, compressive strength, and passive confinement pressure of the cement mortar under impact loads were analyzed to determine the dynamic mechanical properties of the early strength cement mortar under passive confinement. The stress characteristics of the stemming material in the blasting hole under the pressure of the blasting gas were also analyzed to provide a reasonable stemming length for large-diameter deep-hole blasting. The results showed that the self-shrinkage of the test piece increased with the increase of admixture content. The compressive strength of the test piece increased with the increase of age and decreased with the increase of admixture content. With the increase of admixture content and age, the expansion pressure increased, resulting in an increase in passive confinement pressure. When the admixture content was 4%, and the age was 16 hours, the expansion pressure reached its peak at each measuring point. Additionally, with the increase of admixture content, the porosity of the slurry increased, the axial compressibility of the blocking material increased, and the acting time of the blasting gas pressure in the blasting hole was prolonged, which improved the stemming quality. Comprehensive on-site test results have shown that the stemming length, reasonably selected based on the derived theoretical formula and combined with on-site production, ensures safe production during the mining period, and achieves good results in deep-hole blasting.