The invention relates to a device and a method for generating microwave signals by using a multi-wavelength Brillouin laser. An existing device obtains a high frequency difficultly. The device of the invention comprises an adjustable narrow-band light source, two isolators, three couplers, an erbium-doped optical fiber amplifier, two optical fiber circulators, a Sagnac loop mirror, an optical fiber Bragg grating and a photodetector. The adjustable narrow-band light source is in optical connection with the isolators; the isolators are in optical connection with the couplers; outputs of the couplers are in optical connection with another coupler respectively; one of optical paths is provided with the erbium-doped optical fiber amplifier, the two optical fiber circulators and the Sagnac loopmirror. The method for generating the microwave signals by using the device comprises the following steps that: pump spectrum light emitted by the adjustable narrow-band light source is split into two paths of pump spectrum light after passing through the isolators and the couplers, one path of the pump spectrum light performs beat frequency with the other path of the pump spectrum light after outputting single-channel high-order Stocks light so as to obtain high-frequency microwave signals. The device and the method change the wavelength of the pump spectrum light to make a multi-wavelength pass band continuously adjustable.