Abstract The CAS phase (CaAl4Si2O11) is a key phase formed in subducted silica‐alumina rich crustal and sedimentary materials under the mantle transition zone conditions. In this study, its elastic properties were investigated using first‐principles calculations up to 30 GPa. The obtained elastic moduli are: KS0 = 211.24(174) GPa and G0 = 129.61(118) GPa, with pressure derivatives KS′ = 4.33(13) and G′ = 1.49(6). Based on these results and relevant mineral thermoelastic data, we simulated the density and seismic velocity profiles of subducted anorthosite crust. Our simulations reveal that anorthosite crust exhibits higher density and seismic velocities than pyrolite within the mantle transition zone. However, this relationship reverses when ringwoodite in pyrolite decomposes into bridgmanite and ferropericlase. The resulting buoyancy effects suggests that subducted anorthosite crust is likely trapped at the base of the mantle transition zone, which may contribute to the observed high‐velocity anomalies in this region.