Multi-walled carbon nanotubes (CNTs) deposited by chemical vapor deposition (CVD) were treated by ball-milling with different time, the morphology and structure of which were explored with scanning electron microscope and Raman spectra. The field emission properties of the cathode made of CNTs under different ball-milling conditions were tesed. The results showed that, high-energy ball milling brought obvious effects on the morphology, structure and field emission property of the CNTs. A ball-milling time of 0.5～1.0 h resulted in a more even and uniform surface of the CNTs, enhanced the field emission properties, and also greatly increased uniform luminescent spots on the anode. The field emission properties can be degraded if the milling time is too long (>1 h), due to the resulting amorphism and graphitization of CNTs.