The paper aims to improve the structure of Al-20Si alloy by adding Mg to improve its surface mechanical properties. In this experiment, a series of tests were carried out with field emission scanning electron microscopy (FESEM), microhardness tester, and a multi-functional surface property tester to study the surface microstructure and surface mechanical properties of Al-20Si alloy surface treated by HCPEB modified by adding Mg element. The Mg could refine the primary silicon phase by forming finer Mg2Si phase with the Silicon phase, while Mg element could eliminate the microcracks on the Al-Si alloy surface which formed after HCPEB treatment. All the diffraction peaks were broadened and shifted after the material surface was modified by HCPEB treatment. The Al matrix microhardness of these two alloys gradually increased with the increase of the pulse number. The microhardness of Al-20Si alloy Al substrate increased from 745.5 MPa (for initial sample) to 2170.7 MPa (sample treated with 25 pulses), the microhardness of Al-20Si-5Mg alloy Al substrate increased from 1061.3 MPa (for initial sample) to 2403.6 MPa (sample treated with 25 pulses). Meanwhile, the addition of Mg improved the hardness of Al-20Si alloy. In addition, reciprocate fraction test showed that both Mg and HCPEB could improve the wear resistance of material. It can be summarized that adding Mg element can modify the microstructure of the Al-20Si alloy after HCPEB treatment, and improved the surface mechanical properties of Al-20Si alloy treated by HCPEB.