Abstract: Collapse failure is one of the major failure types of oil casing operation.With the practice of gluing strain-gauge on the outer surface of the casing, it is possible to study the strain variation during the casing collapse failure process.Introduced here in the article is an underwater strain-measurement method as used in the casing collapse test.Thanks to the said underwater measurement technique, difficulties like insulation protection of the strain gage under hi-pressure water and sealing of the conductor as going out of the hi-pressure vessel, etc.are over come, and thus regularity of the strain of the casing outer surface vs exterior pressure variation is successfully identified, which will benefit the approach to the collapse mechanism of the casing.