The utility model discloses a high precision on-line monitoring sensor for AC leakage current, comprising a shielding box with a center through hole and a shielding box cover assorted with the shielding box. A coaxial cable connector which serves as signal output is arranged on the shielding box. Insulating film, slope film alloy film and steel sheet film are pasted from the inner wall to the center in the shielding box in turn. A ring shaped core is arranged in the middle of the shielding box, and the ring shaped core is winded with coils. Both ends of the coil are connected with integrator resistors, and both ends of the integrator resistor are respectively connected with the core wire of the coaxial cable connector on the shielding box and the casing line. The sensor can be used for on-line monitoring the AC leakage current and the dielectric loss tangent of the power equipment with AC leakage current of mu A and the 50Hz AC industrial frequency signal with the measuring range of 1 mu A-2mA. The power equipment earth connection directly penetrates the center of the device and monitors, realizing the electrical isolation, and the high precision on-line monitoring sensor for leakage current does not influence the normal operation of the equipment.