Abstract: Neonicotinoid pesticides, including imidacloprid, acetamiprid, and thiamethoxam, are widely used in agricultural production to kill pests by disrupting signal transduction within organisms. Imidacloprid has the properties of high solubility and a long half-life after entering the aquatic environment, and its residues pose potential risks to ecological safety and human health. Therefore, it is necessary to develop efficient technologies to eliminate these contaminants. Here, imidacloprid is selected as the target pollutant. Photochemical simulation experiments and photoactivated periodate (UV/PI) technology were conducted to evaluate the effects of various water environmental factors on the degradation efficiency of imidacloprid and identify the major reactive species involved in the degradation process. Based on the results obtained from high performance liquid chromatography-mass spectrometry (HPLC-MS), the degradation products and pathways of imidacloprid were projected, and the ecological risks of imidacloprid and its degradation products were assessed. The results indicated that the degradation of imidacloprid in the UV/PI system followed first-order reaction kinetics. The degradation performance was optimal when the concentration of PI was 5mmol/L, with a degradation rate of 88.5% within 90min. The effect of PI concentration on the degradation rate followed a trend of initially increasing and then decreasing. A slightly alkaline condition (pH=8) and NO₃⁻ promoted the degradation of imidacloprid, while fulvic acid inhibited the degradation. Singlet oxygen (¹O₂) and iodine radicals (IO₃·, IO₄·) were the primary reactive species to degraded imidacloprid. The degradation pathways mainly involved dechlorination, hydroxylation, and dehydration amidation reactions. The degradation products included imidacloprid guanidine, 6-chloronicotinic acid, and 6-chloronicotinamide. After cleavage of groups such as the imidazole and nitroimine group in imidacloprid, acute toxicity, developmental toxicity and mutagenicity of some degradation products were reduced. However, the incomplete mineralization of these products still had ecological risks. Compared with traditional methods such as physical adsorption and microbial degradation, and other periodate activation methods such as ultrasonication and activated carbon, the degradation of imidacloprid using UV/PI technology in this study exhibits advantages including high reactivity, short reaction time, cost-effectiveness, and environmental friendliness. The findings provide valuable references for the prevention and control of water pollution caused by neonicotinoid pesticides. The BRIEF REPORT is available for this paper at http://www.ykcs.ac.cn/en/article/doi/10.15898/j.ykcs.202503140043.