The invention discloses a sapphire microwave resonant cavity. The sapphire microwave resonant cavity comprises a metal shielding cavity body with an opening, a metal cavity cover matching the opening of the metal shielding cavity body and an annular sapphire crystal. The annular sapphire crystal is fixedly arranged on the metal cavity cover and is located inside the metal shielding cavity body. The metal cavity cover is fixedly arranged at the opening of the metal shielding cavity body. An inlet and an outlet are arranged in the metal shielding cavity body. The metal shielding cavity body is mainly used for shielding influence of an outside electromagnetic field to the microwave cavity frequency, and can be made of metals of copper, aluminum and the like. The annular sapphire crystal has an extremely low loss tangent value in a high frequency band, and an echo wall mode can be generated through reasonable design. The mode has a high Q value, because the annular sapphire crystal material is employed, some normal TE and TM modes are filtered, and thus the microwave cavity has a wide mode spacing. The metal cavity cover is used for fixing the annular sapphire crystal.