Abstract: At present, human activities are gradually increasing, and farmers’ activities have become a hot issue in research of protecting cultivated land and cultivated landscapes. The study encompassed survey data from 540 farmers in Jianping County, Liaoning Province to explore the impact of farmers’ cognition on the biodiversity of surface arthropods in farmland landscapes. Furthermore, it also examined the mediating effects of landscape heterogeneity. By constructing a mediation effect model, the logical relationship between farmers’ cognition, agricultural landscape heterogeneity, and surface arthropod diversity was studied aiming to provide a theoretical basis for landscape protection of farmland. The results showed that: 1) Differing cognition of farmers had varied effects on the diversity of surface arthropods in cultivated landscapes. Farmers’ cognition of pesticide and chemical fertilizer application, agricultural machinery, and ecological characteristics of cultivated landscapes had positive impacts on surface arthropod diversity. The better their cognition, the higher the diversity of surface arthropods in the farmland landscape is. Cognition of scale management and farmland protection policies had a negative impact. This manifested as better cognition leading to lower arthropod diversity. This conclusion differed from that which was expected. This further confirmed the important influence of farmers’ “effect expectations” on behavior. 2) Agricultural landscape heterogeneity had a significant mediating effect on farmers’ cognition affecting surface arthropod diversity. Farmers’ cognition could indirectly affect the diversity of surface arthropods by influencing the heterogeneity of agricultural landscapes. Based on this, the government should increase investment in rural education, build knowledge about agro-ecological protection, and improve awareness among farmers about cultivated land protection. It is also important to pay attention to optimization of cultivated land landscape patterns, appropriately increase non-farming habitats, to ultimately protect surface arthropod diversity, and enhance the ecological benefits of cultivated land.