The utility model discloses a cement dilatometer. The cement dilatometer comprises a cement sheath assembly and an end cover structure, wherein the cement sheath assembly is used for containing set cement and comprises an inner ring and an outer ring, and an annular cavity structure between the inner ring and the outer ring and is used for containing the set cement; the outer ring is provided with a detection structure which is used for detecting the expansion degree of a cement sheath; the detection structure has a closed state and an open state and is changed into the open state from the closed state when the cement sheath expands; the end cover structure is used for closing the annular cavity structure and comprises an upper end cover and a lower end cover, and the upper end cover and the lower end cover are arranged at two ends of the annular cavity structure. The cement dilatometer can be applied to the technical field of petroleum well cementation; by adopting the cement dilatometer, a high-temperature and high-pressure maintenance environment under a well can be actually stimulated, and the expansion performance of a set cement system can be effectively monitored.