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The precise boost pressure control is the key to guaranteeing reliable turbocharged gasoline engine operation. The closed-loop control requires fast boost pressure tracking performance in wide engine operating range. This paper presents a double closed-loop nonlinear control scheme to improve transient tracking performance. The air-path system of the turbocharged gasoline engine is described by a physics-based, nonlinear, mean value model that presents a long regulation channel from the control input, waste-gate-opening, to the control output, boost pressure and brings challenges for the controller design with satisfied tracking performance. To address this problem, the outer loop treats the turbine speed as a downstream control demand and adjusts it by using a feedforward-feedback control to achieve fast boost pressure tracking. Subsequently, a disturbance-observer-based nonlinear inner-loop controller is designed to regulate the wastegate that stabilizes the turbine speed tracking error while handles the uncertainties due to operating variation. The stability of the whole closed-loop system is analyzed by the Lyapunov stability theory and the robustness against operating variation is discussed based on input to state stability (ISS). The effectiveness of the proposed controller is validated through a higher fidelity model in AMESim and its performances are compared with well-tuned baseline controllers.