Conventional microporous adsorbents suffer from CO2 adsorption capacity reduction due to the presence of water vapor in real flue gases. Therefore, development of low-cost moisture-insensitive adsorbents is of great significance. In the present work, microporous copper silicate was synthesized from waste fly ash for the first time. The synthesis conditions were further optimized to obtain copper silicates with high purities. The as-synthesized product was characterized in terms of structural morphology, chemical composition, and surface properties. The CO2 and H2O adsorption properties of the prepared copper silicate were also investigated by single-component isotherm measurements and dynamic CO2/H2O binary breakthrough experiments. The results show that the as-synthesized fly ash-based copper silicate exhibited excellent CO2 adsorption properties even in the presence of water vapor, thus demonstrating promising potential as a moisture-insensitive adsorbent for directly capturing CO2 from humid flue gases.