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The cracking and reasons for aging of silicone rubber current transformer insulation sheath from a 500 kV substation were investigated. Chemical characteristics including attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, mechanical properties, and thermo gravimetric analysis (TGA) have been performed on the external surface and internal parts of the silicone rubber CT insulation sheath. The FTIR results indicated that the external surface of the silicone rubber CT insulation sheath suffered from more serious aging than the inner, fracture of the side chain (Si-C bond) was much more than the backbone (Si-O bond). Mechanical test results revealed that the tensile strengths of the external surface parts of the silicone rubber CT insulation sheath were lower than those of the internal parts. TG analysis data indicated thatTonsetdropped nearly 50°C for the external surface of the silicone rubber CT insulation sheath. All the chemical characteristics have showed that the external surface material of the silicone rubber CT insulation sheath had been serious aging. Our work suggested a facile and effective method for evaluating the aging state of the silicone rubber insulation sheath in the electric power field from the perspective of chemical. [ABSTRACT FROM AUTHOR]