The invention discloses a method of synthesizing L-carnosine using L-amino acid ligase by a one-step method. The method comprises the steps of taking beta-alanine, L-histidine, ATP (adenosine triphosphate) and polyphosphate as raw materials, and MgCl2 as an activator, adding L-amino acid ligase and polyphosphate kinase, and synthesizing L-carnosine through an enzymatic reaction and coupled coupleunder conditions of a pH of 6.5-8.5 and a temperature of 30-45 DEG C. According to the method, a sequence optimized L-amino acid ligase gene is successfully expressed in escherichia coli, and beta alanine and L-histidine can be catalyzed to synthesize L-carnosine at one step. A substrate is not required to be methylated or subjected to group protection; ATP required by a catalytic reaction is continuously regenerated by allowing polyphosphate kinase to catalyze phosphorylation of ADP (adenosine diphosphate); cyclic regeneration of ATP can be achieved only by consuming a small amount of ADP; and a renewable raw material, sodium hexametaphosphate, is wide in source, low in cost, simple to operate and easy in large-scale production.