The present disclosure is related to the microwave measuring field, and in particularly to a coaxial resonant cavity and system and method for measuring the dielectric constant of material. The coaxial resonant cavity includes a coupling mechanism and a cavity body. The coupling mechanism is accommodated in the cavity body for exciting or coupling microwaves inside the cavity body. The coaxial resonant cavity further includes a probe extending out of the cavity body and being coaxial with the cavity body. The cavity body is shaped as an annular column, and a ratio of an outer radius of the annular column to an inner radius of the annular column is (3-5):1. The present disclosure still provides a system and method for measuring the dielectric constant of material using the coaxial resonant cavity.