The present disclosure relates to the technical field of surveying and mapping building structures, in particular to a built-in gap measuring device and a gap measurement method. The built-in gap measuring device includes an adjustment mechanism and measuring mechanisms disposed on both sides of the adjustment mechanism. The adjustment mechanism can adjust the distance between two groups of measuring mechanisms, and the measuring mechanism includes a panel structure and a measuring column. The panel structure includes a bottom cover close to the adjustment mechanism and a top cover away from the adjustment mechanism. A plurality of rows of elastic column holes are arranged on the top cover in array, column seats in one-to-one correspondence with elastic column holes are arranged inside the panel structure in array.