The invention relates to the field of measuring instruments, in particular to an external thread pitch diameter measuring device, comprising a measuring ruler, a convex measuring head and a convex measuring head; the concave measuring head comprises a measuring head seat provided with first and second slip jaws which cooperate and form a measuring tooth slot for clamping a thread tooth, the measuring tooth slot includes first and second clamping faces for clamping the thread tooth, the first clamping face is arranged on the first slip jaw, the second clamping face is arranged on the second slip jaw, the first and second slip jaws are in guided sliding fit with the measuring head seat, the size of the measuring tooth slot may be adjusted by sliding the first and second slip jaws, the convex measuring head is provided with measuring teeth paired to the measuring tooth slot and used for matching with a thread tooth groove. The size of the measuring tooth slot in the external thread pitch diameter measuring device may be adjusted to adapt to threads of different pitches, and the problem that an existing external thread pitch diameter measuring device has a need for changing measuring heads to measure external threads of different pitches is solved.