The invention belongs to the technical field of nondestructive detection and radiographic testing, and particularly relates to an optical densitometer for automatically measuring density range. The optical densitometer for automatically measuring the density range comprises a light source control system, a light source receiving device, a track conveyor and a central control system, wherein the light source control system and the light source receiving device are oppositely arranged; a plurality of photographic films are arranged between the light source control system and the light source receiving device and positioned on a conveying track of the track conveyor; the central control system is correspondingly and electrically connected to the light source control system, the light source receiving device and the track conveyor. The invention also provides optical density calculation method. According to the optical densitometer for automatically measuring the density range and the optical density calculation method, the problems such as low efficiency, high working intensity, large measuring error and lack of standardization of a traditional densitometer at present can be solved; the concept of a linear sensing array is introduced and free from a single point sensor is avoided, so that the automatic measuring efficiency of a complete photographic plate can be greatly improved.