The invention provides a gate valve body seat ring hole detecting device, which is characterized in that a first tapered wedge and a second tapered wedge rotate by taking a steel ball as a mass point when the seat ring hole detecting device gets into the internal part of a gate valve body required to be detected so as to be compressed and attached to two seat rings adjacent to the first tapered wedge and the second tapered wedge respectively. Measuring shafts are fixed at the top ends of the first tapered wedge and the second tapered wedge respectively, so that the measuring shafts move in a mode of being synchronous to the first tapered wedge and the second tapered wedge. An x-value acquired by measured can reflect the size of an included angle alpha between the two seat rings inside the gate valve; and a y-value can reflect the open size b between the two seat rings. Therefore, the acquired x-value and the y-value can quantitatively reflect deviation between the processing size and the theoretical size of seat ring holes of the measured gate valve body, thereby being conducive to playing a guiding effect for field processing.