The invention discloses an extensometer testing system used in a liquid environment. The testing system comprises a U-shaped fixture located in an environmental box, liquid such as an acid-base solution is added into the environmental box, and an extensometer is composed of two anti-corrosion arm parts which are movably connected in a riveting mode. Clamping heads are attached to the ends of short arm portions, located in the environmental box, of the anti-corrosion arm parts and used for clamping an experiment part, and pointers are attached to the ends of long arm portions, located outside the environmental box, of the anti-corrosion arm parts and used for pointing scales of a measuring meter. The extensometer takes a rivet arranged at the riveting joint as the pivot, and according to the lever principle, deformation measured by the short arm portions is amplified by the long arm portions, pointed on the measuring meter and collected by a CCD image collecting system. Beginning and end data of the CCD image collecting system is read, and then deformation of the tested position of the experiment part in the whole experiment process can be accurately acquired.