The invention discloses a nano catalyst with high catalytic activity for two common electro-catalyzed reactions: oxygen reduction reaction and oxygen evolution reaction, and a preparation method thereof. The catalyst is composite catalytic Pd/Ni-Mo/C which takes conductive carbon as a carrier, is loaded with a NiMo alloy and is partially coated with a precious metal Pd. The preparation method comprises the following steps: loading a certain ratio of NiMo on the surface of a carbon carrier by using a dipping-high-temperature reduction method so as to obtain NiMo/C, and selectively partially coating NiMo/C with a certain ratio of the precious metal Pd by using an ethylene glycol reflux method. On one hand, under the mutual action of a transition metal and the precious metal, the electron structure on the surface of the precious metal is changed, and the oxidation reduction activity of the catalyst is improved; on the other hand, since a non-precious metal alloy is exposed in a limited manner, the oxygen evolution activity and the methanol poisoning property of the catalyst are improved to a certain extent; furthermore, the surface utilization rate of Pd is increased since NiMo is loaded with Pd; the Pd/Ni-Mo/C bifunctional catalyst with high activity, positioning prevention and precious metal conservation has important research significances for reducing energy losses and costs of rechargeable fuel batteries and metal-air batteries.