Due to the complex crystal structure for low lattice thermal conductivity and the tunable valence bands for superior electronic performance, CaAl₂Si₂-structured AB₂C₂ Zintl compounds have been frequently proven as promising p-type thermoelectric materials. In this work, thermoelectric properties of CaMg₂Bi₂ are systematically investigated in a broad carrier concentration (10¹⁸–10²⁰ cm⁻³) through Ag-doping for comprehensively evaluating its potential for thermoelectric applications. The broad carrier concentration enables a well assessment of the carrier transport properties by single parabolic band with acoustic phonon scattering and a revelation of the carrier transport by multiple valence orbitals when the carrier concentration higher than ~2 × 10¹⁹ cm^–3, leading to a significant enhancement in electronic performance. With the help of additional point defect phonon scattering introduced by BaMg₂Bi₂-alloying, a reduction in lattice thermal conductivity in the entire temperature range and the lowest one of ~0.7 W/m-K are achieved, leading to a 100% enhancement in average zT_ave. in addition to the contribution of a multiband transport. This work not only demonstrates CaMg₂Bi₂ as a promising thermoelectric material, but also provides a well understanding of its underlying material physics.