The invention relates to a device and method for quantitatively charging lime automatically, belonging to the technical field of wastewater treatment. The device comprises a lime storage bin, a blanking machine, a feeding machine, a lime milk preparing tank, a preparing pump, a batching pool, a batching pump and a neutralizing pool. The method comprises the following steps of: pumping lime which is in bulk in a tanker into the lime storage bin; quantitatively conveying the lime in the lime storage bin to the lime milk preparing tank for digesting by the blanking machine and the feeding machine in sequence; inputting acid wastewater into the lime milk preparation tank; quantitatively pumping the digested lime milk into the batching pool by using the preparation pump; and quantitatively charging the processed lime milk into the neutralizing pool for carrying out acid-base neutralization by using the batching pump. The invention has the advantages of full-closed design, high automatic level, low labor intensity, accurate lime charging amount and stable running.