Abstract: Studies on interactions between plants and belowground organisms or between plants and aboveground organisms are numerous. However, research on the effects of belowground organisms on aboveground organisms via plant mediation remains limited. Soil organisms can influence interactions between plants and phytophagous insects‌ by altering plant growth and chemistry. Four treatments, including addition of 0 (−E), 1 (+1E), 2 (+2E), and 4 (+4E) Metaphire guillelmi individuals per pot, were applied, and three species wheat aphids (Schizaphis graminum, Sitobion avenae, and Rhopalosiphum padi) were inoculated to investigate the effects of earthworms and aphids on the primary and secondary metabolites of wheat, as well as on the population dynamics of three aphid species through indoor potted experiments. This study aims to provide theoretical support for understanding interactions among soil biota (earthworms), wheat, and aphids in agricultural ecosystems. The results indicated that earthworm addition at different densities increased the biomass of wheat shoots and roots, and the total flavonoid and total phenol contents of wheat shoots; whereas decreased the ratio of total carbon to total nitrogen of wheat shoots and roots. The +4E treatment increased the total nitrogen content of wheat shoots and roots, +1E and +2E treatments decreased the root total carbon content. Aphid inoculation reduced the wheat shoots biomass, content of total nitrogen of wheat shoots and roots, whereas increased the ratio of total carbon to total nitrogen of wheat shoots and roots, the total flavonoid under −E treatment and total phenol contents under +4E treatment of wheat shoots. The population dynamics of all three aphid species exhibited logistic S-shaped curves, and their population densities and environmental capacity (K) increased with rising earthworm density. Correlation analysis revealed that wheat shoot biomass, total flavonoid content, and total phenol content were positively correlated with the population density of the three aphid species. As earthworm density increased, the biomass, and contents of total carbon and total nitrogen of wheat shoots also increased. Redundancy analysis showed that biomass and primary metabolites (soluble sugar, total carbon, and total nitrogen) of wheat shoots explained 48.83% of the variation in aphid population size, whereas secondary metabolites (total flavonoids and total phenols) of wheat shoots and soil physicochemical properties explained 11.91% and 31.90% of the variation, respectively. Under aphid inoculation, earthworm addition mitigated the negative effects of aphids on the biomass and total carbon of wheat shoots. These results provide an important basis for understanding the interaction mechanisms among soil, plants, and pests.