The invention provides a water-containing instrument online detection method. A water-containing instrument comprises a signal emitter, a signal detection circuit, a wire and a generator. The generator is internally provided with an inner conductor and an outer conductor arranged at the outer portion of the inner conductor, a medium space is arranged between the inner conductor and the outer conductor, and the medium space is filled with an oil-water mixture medium; two ends of the wire are correspondingly connected with two ends of the generator to form a signal transmission loop; the signalemitter generates a set of 30-100MHZ plane sinusoidal waves as input signals; and the input signals are transmitted to one end of the generator through the wire, and the input signals are transmittedto the other end of the generator through the oil-water mixture medium between the inner conductor and the outer conductor and output signals are generated. The water-containing instrument online detection method achieves detection of a crude oil water content accumulation condition through determination of data value selection and combination of an integrated detection method so as to improve theaccuracy of the water content detection value all day long.