A control device and method for the air system of a diesel engine is disclosed. The feature of the diesel engine is characterized by transfer function. During the control process, a decoupling transfer function is computed according to the transfer function and the steady working parameters of the diesel engine. By the decoupling transfer function acting on the processed state parameters of the air system, driving signals for controlling the exhaust gas recirculation system and the turbocharge system can be individually generated from one another, in order to realize decoupling of them.