Aiming to address issues such as incomplete material fragmentation and poor spreading uniformity in current mechanized solid manure return operations, a high-efficiency spreading machine fragmentation device suitable for large-scale operations was designed. By integrating strength design theory, dynamic analysis, and discrete element method (DEM) simulation technology, an interaction model between the fragmentation device and manure particles was established to analyze the influence mechanism of key operational parameters on fragmentation effectiveness. ADAMS kinematic simulation was employed to optimize the ratio of terminal linear velocity at the hammer tip to fertilizer feed rate (linear velocity ratio). Building upon this, orthogonal experiments and response surface methodology (RSM) were applied to conduct multi-objective optimization of hammer length, width, and thickness, using breakage rate and loss rate as evaluation metrics. Results indicated that the optimal parameter combination was as follows: linear speed ratio of 0.47, hammer length of 177mm, width of 47.5mm, thickness of 4.4mm. To validate the optimization, a prototype was developed and field performance tests were conducted. Field results demonstrated that the optimized crushing device achieved an average crushing rate of 80.77% under typical operating conditions, a 25% improvement over that of conventional methods. The measured average loss rate was 8.58%, meeting operational quality standards and effectively resolving the operational challenges of clumping manure causing blockages and being difficult to crush. This research can provide theoretical foundations and technical references for structural optimization design of solid organic fertilizer spreading equipment.