A new thiocyanic acid-free ruthenium sensitizer, CYC-B29, containing two unsymmetrical ancillary ligands, was synthesized, and its three diastereoisomers CYC-B29-CC, CYC-B29-TT, and CYC-B29-CT with significantly different optical, electronic, and electrochemical properties were carefully separated. CYC-B29-TT with the smallest size has the strongest absorption coefficient of the MLCT band, the shortest λmax, the lowest highest occupied molecular orbital level and the highest dye loading. Therefore, dye-sensitized solar cell based on CYC-B29-TT has the highest efficiency, which is two times higher than that of CYC-B29-CC-sensitized device and 10% higher than that of N719-based cell. Time-dependent density functional theory-calculated transition bands for the three isomers are not identical, and only CYC-B29-TT has the calculated transition bands close to the experimental absorption profile. Although the calculated transition bands for CYC-B29-CC and CYC-B29-CT are not consistent with the experimental data, the ground-state vertical excitation energy with oscillator strength and electron-density difference map data combining with the dye loading predict correctly the order of the photocurrent for the three isomers sensitized devices.