In this paper, a novel one-dimensional matching method of an asymmetric twin-scroll turbine (ATST) with a small scroll bypass wastegate is initially presented for energy improvement. The developed method presents further insights into efficiency prediction of the ATST and the small scroll exhaust bypass in the matching process of model characterization. The efficiency of the small and large scroll turbines was approximately assessed with two times flow parameters of the small and large scroll turbines, respectively, as well as according to turbine efficiency prediction curves. Subsequently, given the matching results of a 9-L engine, a targeted ATST was developed; its effectiveness was verified by computational fluid dynamics (CFD) and the performance tests of a turbine and an engine. As revealed from the results, the prediction efficiency of the ATST well complies with that of the numerical calculation and performance tests of turbines and engines. Compared with the common large scroll exhaust bypass wastegate, the small one exhibits better engine performance and can save nearly 0.5–1.5% fuel consumption at middle and high engine speeds. Moreover, the reasons of which were explored for better understanding of the mechanism accordingly.