Abstract: The relationship between electronic structrucs and stabilities of transition metal triple-decker sandwich complexes was analysed and discussed.By the use of the level of metal d_xz (or d_yz) orbital in combination of the fragment CpMM'Cp as a standard value, if the HOMO πlevel in the penta- or hexa-coordination ring is near to or higher than this value, the complex would exist stably; on the contrary, if this π level is lower than this value,the complex would not exist stably. The results arc satisfactory for 8 different middle coordination rings. The effects of substituent group in the middle coordination ring are also discussed. We hope that these conclusions are able to guide further the syntheses of transition metal triple-decker sandwich complexes.