A continuous method (P) for preparing diamine is described. The method includes reacting the corresponding alkene nitrile with the corresponding monoamine in order to form the corresponding aminonitrile. The monoamine can be introduced in molecular excess with respect to the alkene nitrile, wherein the unreacted monoamine is recirculated to the reaction; followed by reducing the aminonitrile produced by hydrogen in the presence of at least one alkali-metal hydroxide, water, and a hydrogenation catalyst; and purifying the diamine.