The invention relates to a crude oil electric field action part which is installed on oil pipelines and consists of a cavity, an electric field action area and a power source input terminal; wherein, the cavity is provided with an oil inlet end and an oil outlet end; the oil inlet end is used for being matched and connected with an upstream oil pipeline and for receiving upstream crude oil, while the oil outlet is used for outputting passing crude oil downstream; the electric field action area is arranged between the oil inlet end and the oil outlet end and used for exerting action on the passing crude oil with the electric field; the power source input terminal is arranged on the cavity and is used for inputting voltage which can generate the electric field into the electric field action part; wherein, the ratio of the size of a cross section of the cavity of the electric field action part vertical to the flow direction of crude oil to the size of a cross section of the oil pipeline matched with the cavity vertical to the flow direction of crude oil is from 0.7 to 3. The invention also provides a crude oil dehydration unit and a crude oil dehydration system which includes at least one crude oil dehydration unit. The crude oil electric field action part can lead to more simple and compact structure of a crude oil dehydration and desalting device and can improve the effect and efficiency of crude oil dehydration.