The relationship between the design parameters of core material (including material, density, and slotting method) and mechanical properties of sandwich composites for wind turbine blades was investigated. The results indicate significant differences in the cellular structure of polyethylene terephthalate (PET) and polyvinyl chloride (PVC) core material. The cellular walls of PET core material are highly regular, while those of PVC core material exhibit microcellular structures. Furthermore, PET core material demonstrates superior mechanical strength and modulus, while PVC core material offers enhanced toughness. For PET core material, an increase in density correlates with higher mechanical strength and modulus, albeit at the expense of reduced toughness. The study also examined the impact of various slotting methods on the four-point bending performance of sandwich composites. The results suggest that the slotting method of deep cross grooves on the upper surface and shallow straight grooves on the lower surface may enhance the load-bearing capacity of sandwich composites during bending. Notably, the sandwich composites fabricated with PVC core material exhibit excellent comprehensive performance measured in four-point bending test, which can be attributed to the inherent toughness of PVC core material and the reinforcement ribs. Additionally, the microporous structures present in cellular walls of PVC core material may contribute to better coupling with epoxy resin during infusion process, further enhancing the properties of sandwich composites. This study provides theoretical guidance for optimizing the structural design of sandwich composite materials for wind turbine blades.