The utility model relates to an automatic insulated paper acid number device for testing transformer, the device include control module, spectrophotometer and AD converter, spectrophotometer output is connected with control module through the AD converter, place the cell that holds the sour color development solution of insulating pattern extraction that awaits measuring on spectrophotometer's the magazine. Contain low molecule water -soluble acid according to insulated paper for the transformer, its different acid numbers can demonstrate the principle of different colour wavelength when adding specific colouring agent, detect out these wavelength through spectrophotometer, and then send these wavelength electric signal transition to control module for data signal through the AD converter, carry out autoanalysis by control module to ripples long signal and calculate the back, reach the trial value, to surpassing the value of setting, can send slight ageingly respectively according to surpassing the degree, the warning that well degree is ageing or serious ageing. The utility model discloses do not have other any chemical reagent except that the colouring agent of adding the minute quantity, have the advantage that is showing of high efficiency, simple, environmental protection etc..