The invention discloses a method for preparing piperacillin acid. The method comprises the following steps: adding ampicillin, water and a buffer solution with the pH of 6.0-9.0 into a reactor; adding EDPC into the reactor, meanwhile, adding an alkaline regulator to control the pH to be 6.0-9.0, and reacting for 30-60 minutes in the temperature range of 0 to 10 DEG C while carrying out heat preservation; and adding a solvent to crystallize, controlling the crystallizing point to be 15+/-2 DEG C, dropwise adding an acidic regulator to regulate the end pH to be 1.5-2.0, carrying out crystal growing for 1 hour in the temperature range of 0 to 10 DEG C, and then, filtrating, washing and drying crystals, thereby obtaining the piperacillin acid finished product. According to the method, during acylation, water is used as a solvent, and the buffer solution is added, so that synthetic reaction for piperacillin acid is inhibited from going towards a reverse reaction direction, the yield of piperacillin acid is increased, and the purity of the product is improved.