Abstract: Soluble contamination constituents have significant impacts on the insulation characteristics of outdoor insulator. Deficiencies still exist in traditional compounds detection and ion-pairing method of external insulation for lack of objective quantitative indexes. In this paper, 5 types of insulators with 3 different surface dielectrics are taken as samples, a natural pollution accumulation test lasting for 16 months is conducted. The distribution rules of ESDD, NSDD, chemical constituents and contents of natural pollution on various types of insulator surface are studied. Moreover, aiming at the deficiency of traditional measuring method, we propose a soluble compounds detection method of insulator surface pollution based on FCM (fuzzy c-means). Research results indicate that, ESDD/NSDD and the ratio of NSDD to ESDD of natural contaminated insulators trend to increase from ground terminals to high voltage terminals. The type, structure, shed distribution and surface material of insulators have significant influences on pollution accumulation processes. In Chongqing urban area, the major ionic components of external insulation are SO₄²⁻ and NO₃⁻ for anions and Ca²⁺ for cations. The surface hydrophobicity and hydrophobicity transference enable that the SO₄²⁻ contents of pollution on silicone rubber insulators are obviously higher than that on ceramic and glass insulators. The proposed detection method eliminates the subjectivity in traditional method to achieve sequencing ion-pairing. By applying this method, the major compound components of insulator pollution in Chongqing urban area are measured as Ca(NO₃)₂, CaSO₄ and MgSO₄. In recent years, the mole fraction of nitrates in the tested area increases greatly to the equal level of sulfates.