Abstract: In the oil and gas storage and transportation system of Changqing Oilfield, the oil-water transition zone inthe setting tank gets thicker, leading to high water content in purified oil and affecting normal production. Taking theintermediate layer emulsion produced in the settling tank during dehydration for Changqing Jing'an Oil, with the technicalmeans of microscopy, chromatography, X-ray diffraction and X-ray energy spectrometer, the macroscopic properties ofemulsions and properties of separated water, oil and solid were studied, the formation mechanism of intermediate layeremulsion and the factors that affect its' physical properties were uncovered. The testing results show that the separated waterhas a sulphur content of 35.82 mg/L, total iron of 45 mg/L, the number of SRB in 1 mL water of 10⁴ to 10⁵, the mid-value ofsewage diameter of 85.63 μm, the content of asphaltene and resin of 3.78%, with a high content of FeS in solids. The resultsindicate severe corrosion in upstream gathering system and the serious aging of crude oil in emulsion. In the solids, besidesFeS from corrosion, there are also large amount of minerals such as quartz, feldspar, clay from oil production.