The invention relates to the field of noble metal catalyst synthesis, and discloses a method for preparing chloro(1,5-cyclooctadiene)iridium(I) dimer. The method includes the step that under the oxygen-free condition, an iridium-containing and chlorine-containing compound, aldehyde, 1,5-cyclooctadiene and solvent are subjected to a contact reaction. Traditionally used reactants (alcohol) are abandoned, and new reactants (aldehyde) are used, so the yield of chloro(1,5-cyclooctadiene)iridium(I) dimer can be as high as 75% in short reaction time. Consumption of water and 1,5-cyclooctadiene is low, waste liquid treatment loss is greatly reduced, and production cost is reduced.