In this study, nano calcium peroxide (nCP) was successfully synthesized via chemical precipitation method, which exhibited a large specific surface area (47.98 m²/g) and high purity (84.1%), demonstrating superior performance in degrading naphthalene (NAP) compared to commercial calcium peroxide (CP). Compared with the nCP/Fe(III) system, the chelating effect of citric acid (CA) significantly enhanced NAP degradation, increasing the NAP degradation efficiency from 14.1% to 91.0% within 180 min, indicating a strong promoting effect of CA on NAP degradation. The reactive oxygen species generated in the nCP/Fe(III)/CA system included hydroxyl radicals, superoxide anion radicals, and singlet oxygen, with hydroxyl radicals being the dominant radical responsible for NAP degradation. The superoxide anion radicals indirectly facilitated NAP degradation by accelerating the Fe(II)/Fe(III) cycle. Chloride ions, humic acid, and low concentrations of bicarbonate ions did not significantly affect NAP degradation, whereas high concentrations of bicarbonate ions significantly inhibited NAP degradation in the nCP/Fe(III)/CA system. In groundwater, the adverse effects of environmental conditions can be mitigated by increasing the reagent dosage, demonstrating the potential of the nCP/Fe(III)/CA system for remediating NAP-contaminated groundwater.