The detection technology of effective components in crude oil demulsifier was studied. A generally used demulsifier A with an unknown component was an object. By means of column chromatography and extraction, the components of a demulsifier were separated. Its chemical structure was analyzed by the instrument analysis methods of IR, GC and GC-MS. With acidic methylene blue method, ammonium thiocyanate method and the cloud point method, the type of surfactant in demulsifier A was determined. Using ammonium thiocyanate spectrophotometry, the content of polyether in demulsifier A was quantitative determination. The results showed that the solvent in demulsifier A was xylene, the effective component was propylene glycol polyoxypropylene polyoxyethylene polyether, the effective content was 10.5% (with demulsifier A sample) and 21.8% (with drying demulsifier A) respectively. It contained a small amount of sodium dodecyl benzene sulfonate (0.47%). And the molar ratio of polyoxyethylene and polyoxypropylene segment in polyether was 1.13. In practical application in the oilfield, on the basis of qualitative analysis, the effective component content in a polyether demulsifier might be rapidly detected by ammonium thiocyanate photometric method.