Windshield defrosting performance affects the safety of drivers and passengers in automotives and it is a mandatory testing indicator in most national automotive standards. Therefore the structure design of defrosting ducts is a crucial part of automotive air-conditioning systems. This study focused on the structure of automotive defrosting ducts and used numerical simulation to investigate the defrosting effect and volume allocation of automotive air conditioning. A k-ε turbulence models is established to study the transient temperature distribution of the flow field along the windshield and hence investigates the defrosting effect. By using this established model, the shape of duct and airflow direction at the outlet of the defroster nozzle was optimized. The simulation results showed that the optimized defrosting duct provided the airflow sufficient momentum to blow across the inner surface of the windshield and side windows. It minimized the loss of airflow speed and ensured most of airflow to blow to upper edge of the windshield without detaching from the glass. This optimized design also reduced the air recirculation between the edge of the windshield and dashboard and hence reduced the energy consumption. It substantially improved the defrosting performance.