Turbocharging is the key technique to improve engine-specific power and reduce CO2 emissions of a piston-driven engine. Repeated actions of engine pistons and valves give rise to engine pulsations resulting in intensive unsteady flow in the turbines. Understanding the pulsation effects on a turbine that is steady flow designed is crucially important for performance enhancement. The present work derives an equation that is capable of evaluating the pulsation effects on the volute-outlet flow angle. Based on the equation, a reduced-order model (1D) is proposed and applied to a single-entry mixed flow turbine. By comparing to 3D unsteady computational fluid dynamics (CFD), the ability and accuracy of the 1D model on predicting the unsteady volute-outlet flow angle are verified. The effects of the unsteady flow angle on the turbine performance are further investigated by comparing unsteady and quasi-steady flow-field characteristics given by 3D CFD. The present work unveils the volute unsteady behavior and explains the unsteady coupling mechanism between the volute and the rotor. The findings can lead to improvement of turbine design methodology under pulsating flow conditions.