The invention creatively provides an on-line detection crude oil water content instrument which comprises a meter head, a sleeve, an insulating sealing mechanism, a lead, a welding main body and a generator, wherein the meter head is detachably connected with the sleeve; the welding main body is covered with the sleeve; both the welding main body and the sleeve are of hollow structures; the lead is arranged inside the sleeve; both ends of the lead are connected with the generator through the insulating sealing mechanism; the generator is arranged inside the welding main body; the insulating sealing mechanism comprises a conductive pole, a sealing connector and a sealing welding base; both ends of the lead are inserted into one conductive pole; the conductive pole is sleeved with a conductive pole insulating tube; one end of the conductive pole is clamped inside the sealing connector; the other end of the conductive pole is hermetically connected with the corresponding generator; the sealing connector is clamped inside the sealing welding base; the sealing welding base is fixed with the welding main body. By adopting the on-line detection crude oil water content instrument, the problem of inadaptability of a conventional water content instrument upon liquid water is solved, and due to a good sealing structure, the consequence that danger is caused as a liquid enters the water content instrument can be avoided.