The dielectric loss of the CaCu3Ti4O12 ceramic is high, and the mechanism of the loss is not clear, which restricts its application. The CaCu3Ti4O12 ceramic samples are synthesised by solid state reaction method and coprecipitation method. The electronic relaxation of deep bulk traps at the depletion layer edge, carrier relaxation and the dielectric loss of CaCu3Ti4O12 ceramic are investigated. Both perfect double Schottky barrier and low impurity density can reduce the DC conductivity, thus reducing the low-frequency dielectric loss. High-frequency dielectric loss is controlled by deep bulk trap density, especially in the one whose activation energy is 0.12 eV. At room temperature, when the frequency is 1 kHz, the dielectric constant and loss of CaCu3Ti4O12 ceramic prepared by coprecipitation method are 1.4× 104 and 0.037, indicating a good improvement.