The invention discloses a novel large caliber high sensitivity leakage current detection sensor for a high voltage cable. The detection sensor comprises a closed loop cross-core annular core, measurement coils and a current signal acquisition processing circuit. The fluxgate technology is utilized to design a closed annular core utilizing high magnetic permeability and low coercive force material, a center caliber of the annular core is made not to be smaller than 40mm for penetration of the to-be-detected high voltage DC cable, a four-quadrant symmetric winding method is employed in an excitation coil to wind on the annular core, a response coil is symmetrically wound on the annular core, and the current signal acquisition processing circuit employs closed loop feedback control to form a deep negative feedback circuit. The leakage current detection sensor has advantages of high precision, good stability and strong anti-interference capability, DC leakage currents in a range of hundreds of microampere to tens of milliampere can be measured, sensitivity can reach 100 microampere, and purposes of leakage current measurement and insulation state evaluation of the high voltage DC cable can be realized.