Since the industrial revolution, the concentration of CO₂ in the atmosphere has been rising due to the heavy use of fossil fuel and deforestation. As the oceans and atmosphere exchange gases, the pH of seawater decreases. According to the IPCC, if CO₂ emissions are not effectively reduced, the global surface seawater pH will fall to 7.7 by the 21st century and 7.4 by 2300. Acidification is a growing concern because of the damage it can cause to marine life. In addition, ocean acidification often does not occur in isolation, but often acts on marine life in concert with other stressors. Deltamethrin is one of the most widely used pyrethroid insecticides in the world because of its high activity and photostability to pests. Pyrethroid pesticides have little potential to pollute the environment, but they can enter organisms through the food chain and produce toxicity to aquatic organisms. Previously, pyrethroid residues of 6.36μg/L and 16.2μg/L were detected in coastal areas of Fujian and Zhejiang, respectively. At present, studies on deltamethrin toxicity to aquatic organisms mainly focus on acute and subacute toxicity tests of fish and shrimp, but few studies on shellfish. In order to investigate the toxic effects of seawater acidification and deltamethrin (DM) combined exposure on juvenile of Haliotis discus hannai, three pH values (pH 8.1, pH 7.7 and pH 7.4) and three deltamethrin concentrations (0, 0.6 and 6.0 μg/L) were used to stress juvenile of H. discus hannai. The microstructures and hardness of the shlls, the activity of antioxidant enzymes and the expression levels of genes related to stress and apoptosis in the tissues of juvenile of H. discus hannai were detected. The results showed that after the combined exposure of seawater acidification and deltamethrin for 31 days, the CaCO₃ crystal deposition of juvenile abalone shells was difficult, the aragonite became smaller and formed scattered lamellar stacking, the pores between the aragonite increased, and the hardness of the shells decreased significantly. Meanwhile, seawater acidification and deltamethrin could change the activity of antioxidant enzymes in juvenile abalone. When the pH of seawater was 7.7 and 7.4 or the concentration of deltamethrin was 0.6 and 6, the SOD activity of juvenile abalone was lower than that of the control group, and the SOD activity of juvenile abalone under the combined exposure of seawater acidification and deltamethrin was significantly lower than that of single exposure, indicating that there was an obvious interaction between seawater acidification and deltamethrin on the SOD activity of juvenile abalone. However, deltamethrin significantly increased the activities of CAT and GST in juvenile abalone under the same pH exposure. In addition, this study also observed that the expression levels of stress genes and apoptotic genes in the tissues of juvenile abalone were increased with the increase of pCO2 and deltamethrin concentration, and there was a significant synergistic effect between seawater acidification and deltamethrin exposure on the expression levels of HSP70, HSP90, Caspase-3 and FADD. In conclusion, seawater acidification and deltamethrin exposure can produce significant toxic effects on juvenile abalone and play a certain interactive role in the reaction of tissue oxidative stress and cell apoptosis in juvenile abalone.