<p>In open stoping with subsequent backfill mining, the backfilled stope forms a composite structure (SPS) composed of primary backfill (P) and secondary backfill (S) with different strength ratios (λ), which jointly control the deformation of the surrounding rock. This research systematically investigated strength properties, failure modes, acoustic emission (AE) characteristics, and stress distribution law of the SPS composite with different λ through uniaxial compression experiments. The results indicate that the strength variation characteristic of the SPS composite can be characterized by a modified power function. Cracks in the SPS composite usually initiate at the interface region. When the λ is relatively low, the failure mode of the SPS composite is dominated by tensile failure in the secondary backfill. However, when the λ exceeds 2.32, the SPS composite transitions from tensile failure to mixed tensile-shear failure. As stress approaches its peak strength, the growth of cumulative AE ring counts decelerates, while the percentage of shear cracks rises as λ increases, which exhibits a strong correlation with the failure modes of the SPS composite. By constructing the mechanical model, it was discovered that the stress distribution of the SPS composite exhibits “saddle-shaped” characteristics. The findings can provide a reference for the stability analysis of the backfilled stope.</p>