The invention relates to a quasi-phase-matching higher harmonic device based on ultrasonic modulation in the laser technical field, which comprises a femto-second laser, a focusing lens, an ultrasonic transducer, a bar type nozzle, a metallic film, an X ray spectrometer, a computer, a vacuum chamber, a decompression gas valve and a control panel. The output light path of the femto-second laser issequentially provided with the focusing lens, the ultrasonic transducer, inert gases, the metallic film and the X ray spectrometer. Lasers emitted by the femto-second laser is focused below the bar type nozzle by the lens and interact with the inert gases with density periodic variation which is spouted by the nozzle and modulated by an ultrasonic field to radiate higher harmonics which are reflected into the X ray spectrometer after being filtered by the metallic film and then are sent into the computer. The device meets the quasi-phase-matching condition of the higher harmonics by using ultrasonic modulation gas density, is convenient to be operated and simple and easy, can propel the coherent light source of the higher harmonics soft X ray to shorter waver.