The invention discloses a method for detecting pinhole defects in a photocathode assembly and a realizing device thereof. The method for detecting pinhole defects in the photocathode assembly comprises the following steps that: step one: preparation is made for measurement and the device is initialized; step two: whether a new photocathode assembly is scanned is determined, if the new photocathode assembly is scanned, step three is executed, and if the new photocathode assembly is not scanned but the existing images are required to be viewed only, step five is executed; step three: the movement path of a two-dimensional translation platform is set and the scanning range of the translation platform is set; step four: a CCD camera and an image grabbing card are used to grab images and the original images are loaded to a computer; step five: the images are converted into ideal binary images; step six: regional scanning is conducted to the binary images to find out defect areas and detection results are marked; step seven: seamless jointing of massive pictures is realized; step eight: defect information is integrated; and step nine: scanning is completed. The invention has the advantages of accurate detection results and cost saving.