Based on the feature that outdoor environment has a significant impact on the performance of the heat sink of the thermoelectric cooling system, a numerical simulation study was carried out on the heat sink of outdoor electronic devices, which adopt the thermoelectric cooling system. And the optimum thermal conductance ratio of the heat sink was obtained for the design of the heat sink of the outdoor electronic device. The simulation results indicate that the optimum conductance ratio is 0.42～0.54 when the outdoor monthly average temperature ranges from 0 ℃ to 40 ℃. Besides, the total thermal conductance of the heat sink increases from 30 to 50 W·m/K, and the cooling load undertaken by single TEC is from 20 to 30 W, both of which has little influence on the optimal results.