The invention discloses a crack opening displacement measuring device which comprises a sealed container, a heating unit, a first fixing part, a second fixing part, a first connecting rod component, a second connecting rod component, a first displacement sensor and a second displacement sensor, wherein the sealed container is used for containing a high-temperature corrosion conductive medium; the first fixing part and the second fixing part are used for fixing test samples; the first fixing part and the second fixing part are oppositely arranged inside the sealed container; the first displacement sensor and the second displacement sensor are arranged on the sealed container; the first connecting rod component and the second connecting rod component are respectively connected between a test sample and the first displacement sensor and between the test sample and the second displacement sensor on a thickness cross section of the test sample. By adopting the crack opening displacement measuring device disclosed by the invention, precise measurement measures for testing elastic-plastic cracking toughness and fatigue crack spreading rates of a high-temperature corrosion conductive liquid medium cooling reactor material in complex special environments are provided.