The invention belongs to the technical field of refining of natural ester insulating oil and particularly relates to a device for preparing low-acid-value natural ester insulating oil by an extraction-alkali refining technique. The device comprises an extraction/alkali refining tank, an extraction oil temporary storage tank, an extracting agent storage pool, a cooler, an oil transfer pump and an extracting agent conveying pump; all equipment is communicated with one anther through pipelines. According to the device disclosed by the invention, extracting deacidfieation, extracting agent condensation recovery, alkali refining deacidification and vacuum dehydration technique are carried out in the extraction/alkali refining tank, and occupied area and wastewater discharge are reduced; the device is simple in structure and convenient to operate, and can be used for better preparing the natural ester insulating oil with excellent physical, chemical and electrical properties; the cost is further reduced.