The demulsification and dehydration was more difficulty with the increasing emulsification degree of polymer flooding produced liquid. Taking the produced fluid of polymer flooding of difficult-to-demulsify and normal demulsification in Shengli Oilfield as the research object，the key factors leading to the increase of demulsification difficulty were analyzed by comparing the properties of oil and water samples，cross emulsification and demulsification experiments and microscopic morphology changes of emulsion. At the same time，the influence of internal factors on emulsification was quantified by referring to the theory of colloidal coagulation. The results showed that the interaction between the high valence metal cations and alkylbenzene sulfonate surfactants was the key factor leading to the difficulty of demulsification. In the cross demulsification experiment，the concentration of high valence metal ions was reduced，and the demulsification dehydration rate of crude oil emulsion was greatly improved. The O/W emulsion was prepared with sodium dodecyl benzene sulfonate as emulsifier. After adding Fe3 + ，the stability of O/W emulsion became worse，and it showed a tendency to change to W/O emulsion.The critical threshold of M of Gudao crude oil emulsion was 214 mmol/kg，and the critical threshold of surface tension was 45 mN/m.