The invention relates to a method and device for preparing biodiesel by oil with a high acid value. The method comprises the steps of carrying out acidolysis reaction on the oil with the high acid value and acid solution, then adding methyl alcohol and an acid catalyst in the oil and the acid solution to carry out membrane separation esterification reaction, and finally separating the product and recycling the methyl alcohol, thus obtaining the biodiesel. The device comprises an acidolysis reactor, an oil-water separator, a mixer, a heater, a membrane separation esterification reactor, a metering pump, a buffer tank, a vacuum pump, a methyl alcohol recycling tower and a crude product separator. In the biodiesel preparation process, generated water is timely separated through membrane separation, so that the reaction is rapidly carried out in a positive direction, and the conversion rate of the raw materials is improved; and after the reaction is finished, the catalyst and the methyl alcohol are separated and the catalyst participates in the esterification reaction again, thus the catalyst is recycled and the production cost is saved. According to the invention, the flow is simple, the equipment investment cost is low, the production process is clean, and the environment friendliness is realized.