Solar desalination is gradually emerging as a successful renewable energy source of fresh water. Aiming at the shortcomings of the open circulation solar desalination unit, a closed circulation desalination system based on the humidification-dehumidification principle was designed which can operate 24 h/d. With proper utilization of the latent heat and the circulated airflow, the process efficiency could be increased. In order to optimize the design of this desalination system, a computer simulation program was written in which the thermodynamic models of the components of this system were set up, respectively. This study focuses on studying and analyzing the collector area, cooling water flow rate, seawater flow rate and structural dimension effect on fresh water production. The daily and monthly fresh water production of this system was simulated based on the local monthly solar radiation as an example.