The invention discloses a tunable microwave source, which greatly improves the frequency stability of the microwave source, reduces the phase noise, and tunes the frequency of the microwave source onthe premise of ensuring the stability. The tunable microwave source comprises a microwave oscillation loop, a stable phase system, a filter cavity, an integrator and a microwave antenna. The output ofthe stable phase system is sequentially connected with a filter cavity and an integrator. An integrator and microwave antenna are connected to the microwave oscillation loop. The microwave oscillation loop is used to output microwave signals and radiate microwave signals to the stable phase system through microwave antennas. The stable phase system consists of an atomic gas chamber and a photodetector. The atomic gas chamber is used for inputting a first optical signal and a second optical signal interacting with the alkali metal gas, and outputting the atomic gas chamber radiated by the microwave signal, wherein the first optical signal generates autler-ownes splitting produces a third optical signal. A light detector is use to convert that third optical signal into an electrical signaloutput, which is loaded into the microwave oscillation loop through a filter cavity and an integrator.