The quality of crushing, power consumption, and discharging performance of a straw crusher are greatly influenced by the characteristics of its internal flow field. To enhance the straw crusher’s flow field properties and improve the efficiency with which crushed material is discharged, first, the main structural parameters influencing the air flow in the crusher are discussed. Then, the coupled gas-solid flow field in the straw crusher is numerically calculated through solution of the Navier-Stokes equations and application of the discrete element method (DEM). Finally, the discharge performance index of the crusher is examined through detailed analysis of the crushed material dynamics. Additionally, a multi-island genetic algorithm is used to optimize the structure and operational factors that have significant effects on the discharge performance. With optimization, the accumulation rate of crushed materials in the bottom region of the straw crusher decreases by 20.08%, and the mass flow rate at the discharge outlet increases by 11.63%.