Cd-doping Ba8Ga16CdxGe30-x（x=0.95, 1.00, 1.05, 1.10） type-I clathrates with different Cd contents were synthesized by combining solid-state reaction with spark plasma sintering （SPS） method. The effects of Cd doping on the structure and thermoelectric properties were investigated. Rietveld refinement reveals that Ba8Ga16CdxGe30-x compounds prepared by this method are type-I clathrates with space group pm3n; Cd atoms mainly occupy the 6c and 16i sites in the framework, and the atom displacement parameter （ADP） of Cd is relatively higher than that of other atoms. All specimens show the characteristics of p-type conduction. The carrier scattering mechanism is mainly ionized impurity mechanism in the low temperature range, and gradually changes to acoustical mechanism with the increase of temperature. With the increase of Cd content, the electrical conductivity increases while the Seebeck coefficient decreases gradually. The lattice thermal conductivity of the Ba8Ga16CdxGe30-x compounds is relatively low due to the larger ADP of Cd, and it decreases by about 38% compared with that of Ba8Ga16Ge30 The maximum ZT value of 0.173 is obtained at 600 K for Ba8Ga16Cd100Ge2900 compound.