Abstract: Electroplated diamond tool was prepared by pulse electrodeposition in external alternate magnetic field.The external alternate magnetic field is vertical to the electric field direction.The observation result under SEM (Scanning Electron Microscope)and X-ray diffraction shows that alternating magnetic field is introduced to refine the crystal grain size of the plating, make the Nano Ni-Co alloy coating becomes smooth and compact; The hardness of the plating reaches the maximum value 632HV of 150V alternating magnetic field.The service life of the diamond tool of 150V alternating magnetic field is 1.3 times as that of the tools without magnetic field applied.