The invention discloses a method for reducing 8-benzyl-2,8-diazabicyclo[4,3,0]nonane and a chiral isomer thereof. The method comprises the steps of: taking 8-benzyl-7,9-dioxo2,8-diazabicyclo[4,3,0]nonane or (1S,6R)-8-benzyl-7,9-dioxo2,8-diazabicyclo[4,3,0]nonane as a raw material, and adopting a metal borohydride/BF3 reduction system for reduction to obtain a corresponding product, namely the 8-benzyl-2,8-diazabicyclo[4,3,0]nonane or (S,S)-8-benzyl-2,8-diazabicyclo[4,3,0]nonane. The method adopts the metal borohydride/BF3 reduction system for reduction, avoids the use of an expensive and dangerous reagent, namely lithium aluminum hydride, reduces production cost, improves the safety of the operation, and provides a safe and economical production method for industrial mass production of a moxifloxacin intermediate, namely the (S,S)-8-benzyl-2,8-diazabicyclo[4,3,0]nonane.