The temperature dependent properties and the side leg heat dissipation have serious effects on the output power and efficiency of the thermoelectric generator (TEG). In this paper, a novel steady-state one-dimensional model was developed considering temperature dependence of the properties and the side leg heat dissipation to study TEG performance. The numerical results of the model were compared with the available experimental data and models. The comparison reveals that the deviations for output power and efficiency of TEG are about 3.0 ~ 12.0% and 0.9~17.5% respectively, when the temperature difference between the hot and cold side is within 68 ~ 149 K (The hot side temperature is within 362~452 K). The deviation between the others model and the experimental data exceeds 20% when temperature difference exceeds 103 K. When the temperature difference is above 103 K, the temperature dependent properties and side leg heat dissipation have a pronounced effect on TEG performance. While, the two factors have no obvious influence on TEG efficiency prediction, when temperature difference is below 68 K. The numerical results show that the most influence factor leading to temperature and heat flux non-uniform is the temperature-dependent of material, and followed by Thomson heat and Joule heat.