The invention relates to a device and a method for automatically testing the acid value of insulation paper for a transformer. The device comprises a control module, a spectrophotometer and an AD (analog-digital) converter, wherein the output of the spectrophotometer is connected with the control module through the AD converter; and a cuvette for containing a to-be-tested insulation paper sample extracting acid chromophoric solution is placed on a cassette of the spectrophotometer. According to a principle that the low-molecular-weight water-soluble acid, which the insulation paper for the transformer contains, with different acid values can display different color wavelength when a specific color developing agent is added, the spectrophotometer can detect the wavelength, the AD converter can convert the wavelength electric signals into digital signals and transmit the digital signal to the control module, the control module automatically analyzes and calculates the wavelength signals to obtain test values, and for the test values which exceed a set value, alarm about light ageing, moderate ageing or serious ageing can be sent out correspondingly according to the exceeding degree. Except for a small amount of color developing agent, any other chemical reagents are not added, so the device and the method have the significant advantages of high efficiency, simplicity, environmental friendliness and the like.