The utility model discloses a capacitance sensor of a well mouth crude oil moisture content online measuring device which is applied to the well mouth crude oil moisture content online measuring of the single oil producing well of an oil field. Both a conduit tube and a capacitance tube adopt stainless steel capillary tubes; the surface of the capacitance tube is coated with an insulated coating; the distance between the conduit tube and the capacitance tube is 3-6mm; the conduit tube and the capacitance tube are respectively fixed on an electrode seat; a connecting column penetrates a coaxial hole on a substrate wall; the two ends of the heating wires inside the conduit tube and the capacitance tube are respectively connected with a heating wire; the conduit tube and the capacitance tube are respectively connected with the output electrode wire of a measuring capacitance; a hood cylinder is arranged on the upper part of a round transition block; the hood cylinder and the transition block are fixed by a connecting bolt. The capacitance sensor of the well mouth crude oil moisture content online measuring device has the effects of implementing accurate online measuring on the crude oil moisture content in the well mouth, and being capable of meeting the demands of dynamic analysis of the oil well and also meeting the requirements of remote transmission of data. The capacitance sensor of the well mouth crude oil moisture content online measuring device has a high measuring accuracy; the maximum relative deviation of the capacitance sensor of the well mouth crude oil moisture content online measuring device is within a scope plus or minus five percent; moreover, the capacitance sensor of the well mouth crude oil moisture content online measuring device has small online measuring error and higher reliability.