The decomposition of straw releases nutrients that support crop growth, thereby increasing soil organic matter and fertility. Aiming to achieve rapid decomposition based on the field returning, a tossing-type combination machine was designed for rapid decomposition in the field return of straw. The optimized crank-rocker dimensions were solved based on Graff's theorem and pressure angle conditions, where the length of the crank was defined as one length factor. A model coupling the discrete element method and multibody dynamics was established to simulate the mixing process of liquid decomposition agents and solid phases (soils and straws), utilizing the Kneading contact model. A mixing index was innovatively defined to quantify the mixing performance of the machine in simulations and the field. It was shown that the length factors of crankshaft, connecting rod, rocker arm, frame, and theoretical maximum tillage depth were solved as 1.00, 2.60, 2.60, 3.50, and 1.492, respectively. By using the definition of the mixing index, the calculated mixing indices of the simulation and field were 0.54 and 0.47, respectively, where the relative error between simulations and fields was 12.94%. A systematic technology was proposed for quantifying the solid-liquid mixing, which could take effect simultaneously in both simulation and physical environments. Novel research perspectives and solutions were provided for the mechanization of promoted decomposition in maize straw returned directly.