Abstract: With AM60 magnesium alloy and silicon as raw materials, based on in-situ reaction using high-energy ultrasonic treatment method to improve the conditions, magnesium matrix composites with Mg2Si as reinforcing phase were synthesized. By means of X-ray diffraction and optical microscopy, the phase and microstructure of the composites were analyzed and observed. It was compared with the composites obtained without ultrasonic treatment. The results show that the Mg2Si reinforcing phases generated by the ultrasonic in-situ reaction were smaller and the agglomeration phenomenon was improved. The main reasons of the refinement of reinforcing phase were transient high temperature, high pressure and acoustic streaming effect from acoustic cavitation under the high-energy ultrasonic treatment.