This study explores the cracking gas production characteristics of transformer oil under power-frequency AC voltage. The experiment used a needle-plate electrode structure to generate discharge in mineral oil. Gas chromatography was used to analyze the composition of gas products, and emission spectrum technology was used to analyze plasma characteristics. Results show that at a single-cycle discharge energy of 0.25 J and 0.35 J, the main gas products are H₂, C₂H₂, C₂H₄ and C₂H₆. Among them, the proportions of C₂H₂ and H₂ increase significantly with higher discharge energy. Spectral analysis shows that power-frequency discharge is more conducive to C₂ formation, which further confirms its high-temperature cracking characteristics. The study also calculated electron density and temperature using the Stark broadening method and Wien displacement method. This work provides a theoretical basis and experimental data for transformer fault diagnosis technology.