DescriptionAt present, data centers are widely used in computer software companies, banks and so on. It requires a huge amount of electric power to operate these data centers and the majority part of the electric power is used to cool the computer units in the data center, so it is important to study the cooling system of the data centers. Since data centers are becoming bigger and bigger, it is harder to manage the cooling system of the data center. Air-cooling systems are inefficient in emergency cases. In this research, the influence of air-cooling system performance by inlet temperature, inlet velocity and the heat output of the computer units are examined. A data center with raised floor air-cooling system is adopted. Ansys 12.0 is used to simulate the thermodynamic conditions of the data center at different situations. It is concluded that, the inlet temperature directly affects the maximum temperature of the data center. For every one degree the inlet temperature lowered, the maximum temperature of the data center will be lowered by 0.487 degree. Because it is economically infeasible to lower the inlet temperature to a large extent, it is not a good method to cool the data center in emergency cases. The heat output of computer units is also directly proportional to the maximum temperature of the data center. When the heat output increases by 1000 , the maximum temperature of the data center will be increased by almost 80 degrees. The heat output of the computer units increases several thousand watts per cubic meter in a short period for emergency cases. It is very important to control the heat output of the computer units, otherwise, an air- cooling system just cannot control the temperature. Simply increasing the inlet velocity will not help cooling the computer units. Every computer unit has its own best inlet velocity based on its positions.