Provided are a method for preparing a nitrile-based rubber having an amount of ethylenically unsaturated nitrile of 34 wt % or less including preparing a polymer by performing a polymerization reaction by adding an ethylenically unsaturated nitrile-based monomer in two or more portions to a conjugated diene-based monomer, and coagulating the polymer by adding a sulfur-based coagulant to the polymer, and a nitrile-based rubber thus prepared having improved mechanical properties as well as excellent processability and low temperature characteristics.