The synthesis of AgIn5Te8 is realized and the thermoelectric properties dependent on temperature is examined. However, the mechanism remains unclear and the synergistic effect of the temperature and the carrier concentration on thermoelectric properties is not explored. Here, the electronic transport properties are calculated by using the first-principles density functional theory combined with the semi-classical Boltzmann transport theory. The relaxation time is estimated by the deformation potential theory. The lattice thermal conductivity is evaluated by the Slack's model, and the result is in good agreement with the experimental value. High anisotropic behavior is identified for the thermal and electronic transport properties, which supports the experimental observation. By tuning the carrier concentration and temperature, a maximum thermoelectric figure of merit of 2.28 can be achieved for p-type AgIn5Te8 in xx direction. The present results provide insight into the thermoelectric properties of AgIn5Te8 and a guide to prepare thermoelectric materials with AgIn5Te8.