An electromagnetic/ultrasonic preparation method of an in-situ particle reinforced magnesium-based composite material belongs to the technical field of metallurgy. The invention discloses a method for preparing the magnesium-based composite material with an electromagnetic continuous casting method, which is characterized by comprising the following steps of: melting a magnesium-based melt added with micro-alloying elements of Ca, rare earth Y, rare earth Ce; selecting an Al-Ti-C or Al-Ti-B reinforced system, adopting a self-propagating high-temperature synthesis method to synthesize a magnesium-based composite material melt containing reinforced particles in situ, and implementing electromagnetic/ultrasonic combined stirring to the magnesium-based composite material melt; and finally adopting a continuous casting process to form the magnesium-based composite material by continuous casting, and applying an electromagnetic field and an ultrasonic field within the range of a crystallizer so as to obtain a multi-phase reinforced magnesium-based composite material continuous-casting billet. The electromagnetic/ultrasonic preparation method has the effects and advantages of organically combining the self-propagating reaction method with the electromagnetic continuous casting technology and the ultrasonic technology, obtaining the magnesium-based composite material continuous-casting billet with smooth surface, uniform distribution of particle reinforcement phases in the basal body and good combination of the reinforced bodies and the basal body, and having simple preparation process.