The invention relates to a distributed optical fiber distortion detection tube and detection method thereof, which has a main cylinder with two ends being respectively connected to one port of two auxiliary cylinders via step joints to form a tube chamber, the tube chamber having an inner chamber equipped with optical fiber unit channels composed of fixing clips, a mass block, and step joints, aswell as reserved optical fiber volumes. When being used for detection, the free end of the tube chamber is placed into a target medium, the fixed end of the tube chamber is connected to a distributedoptical fiber sensing analyzer, and as a result, deformation of the medium causes downward inclination or upwarping of the free end of the tube chamber, the mass block in the tube chamber slides toward the end of the tube, the optical fiber in the lower or upper optical fiber channel is stretched, and the signal is detected by the distributed optical fiber sensing analyzer. The technical scheme overcomes attenuation of actual strain of optical fibers due to surface adhesion to their carriers, as well as defects of damage or even break due to being uncoveredly laid on the outer surface of inclination survey tube, thus is applicable to constrained distortion, long-term monitoring, safety control, and professional maintenance of various engineering such as dams, slopes, pit foundations, piles, and cofferdams.