The present invention discloses a microwave signal synthesis device. The microwave signal synthesis device comprises a direct digital frequency synthesizer, an I/Q modulator, a phase-locked loop circuit, a loop filter and a voltage controlled oscillator; the microwave signal synthesis device receives input reference signals and generates microwave output signals; the direct digital frequency synthesizer generates first frequency signals; the voltage controlled oscillator generates the microwave output signals; the I/Q modulator carries out frequency mixing on the first frequency signals and the microwave output signals so as to generate second frequency signals; the phase-locked loop circuit comprises a first frequency converter and a phase detector and is used for carrying out frequency comparison on the second frequency signals and the input reference signals; and the loop filter generates voltage controlled oscillator adjustment signals. The microwave signal synthesis device of the invention is applied to smalls-size rubidium atomic frequency standards. With the microwave signal synthesis device adopted, the problems of complex debugging and poor temperature characteristic of a traditional microwave signal synthesis device can be solved. The microwave signal synthesis device of the invention has the advantages of small size, no debugging, good temperature characteristic as well as excellent phase noise and spurious performance.