La_2O_3 and Y_2O_3 co-doped Mo secondary emitters were prepared by three kinds of doping method combined with high temperature plasma sintering. The secondary electron emission property and microstructure of the cathodes were studied. It showed that the cathode prepared by liquid-liquid doping method exhibited the best emission property among all the samples prepared by liquid-solid doping,solid-solid doping and liquid-liquid doping methods due to a uniform distribution of different substances. RE_2O_3 existed uniformly at the grain boundaries and diffused to the cathode surface during a cathode activation period. A surface layer of RE_2O_3 formed after activation played an important role in the emission. The secondary emission yield of La_2O_3 and Y_2O_3 co-doped Mo prepared by liquid-liquid doping method was 4.34.