A device and method for improving stack performance of a fuel cell system are provided. The device includes a fuel cell controller that operates a stack of a normal-pressure fuel cell system. When the fuel cell controller determines that an operating state of the stack is normal and a current intake air pressure is decreased as a result of monitoring the current intake air pressure, a current output, and an exterior air temperature of the stack, the fuel cell controller increases the amount of air to be supplied into the stack by adjusting a range of a theoretical air ratio which is a theoretical ratio of an air amount to a coolant temperature in the stack.