An electrochemical method for ammonia synthesis including the steps of: preparing a single-crystalline metal thin film; and synthesizing ammonia by using the single-crystalline metal thin film electrode. More particularly, it relates to improvement of the production yield and synthesis rate of ammonia trough the method for preparing ammonia by using an electrochemical reactor which includes a cathode including a single-crystalline metal thin film on the surface thereof, an anode and an electrolyte, wherein the method includes the steps of: supplying nitrogen to the cathode; supplying aqueous electrolyte solution to the anode; and applying an electric voltage between the cathode and the anode.