Field emission characteristics of metals-doped nano-diamond on titanium substrate are studied, wherein metals—Ti and Hf are mixed with nano-diamond separately, and cathode samples are prepared by electrophoresis. Then the structures and morphologies of cathode samples are characterized by SEM and XRD, finally, field emission test is conducted. Results show that the morphology of 5 mg Ti-doped sample is more uniform and dense, but the morphologies of Hf-doped samples become worse. Meanwhile, compared with undoped sample, field emission characteristics of Ti-doped samples improve, Hf-doped samples decrease. Compared between Ti-doped samples, it is found that the amount of Ti mixed with nano-diamond should not be too much, mixing a moderate amount of Ti can make field emission characteristics of nano-diamond on Ti substrate improve. Analysis shows that this is determined by the nature of metal Ti and Hf, which influences the bonding reaction between nano-diamond and substrate Ti, therefore, field emission characteristics of doped nano-diamond on Ti substrate are influenced.