Orthorhombic Ca1−xEuxAlzSiN2+z polycrystalline powders have been synthesized by a solid‐state reaction at high temperature in a nitrogen atmosphere. The smaller lattice parameters compared with the single crystal and charge distribution analysis indicate that impure oxygen might be incorporated into the lattice. Ca1−xEuxAlzSiN2+z shows red emission at about 630 nm due to the transition of 4f65d1→4f7 when excited within 375–450 nm. With an increase in the Eu2+ concentration and z, the quantum efficiency of Ca1−xEuxAlzSiN2+z was significantly decreased due to concentration quenching and high photoionization. First‐principle molecular orbital calculations suggest that the overall luminescence of Eu2+ may be mainly originated from the small Eu(1) center. CaSiN2:Eu2+ is a potential red conversion phosphor for white LED applications.