Prepared Al-15wt.%Mg2Si composite by in-situ, little research had been done in binary phase of Al-15wt.%Mg2Si composite. Employed the method of qualitative and quantitative analysis to confirm the phase components and content by XRD test. Simulated the XRD Patten of Al-Mg-Si alloy by Reflex and compared the simulated result with testing result. Lattice constants, thermodynamic parameter and elastic constant of Mg2Si phase and Mg17Al12 phase in Al-Mg2Si composite were investigated by means of first-principle calculations from CASTEP program based on density functional theory, in order to compare the stability and mechanical properties. There are only α-Al phase and Mg2Si phase in Al-15wt.%Mg2Si composite shown in XRD testing result; and the mass fraction of phase Mg2Si is 14.9%. The differences of XRD indices with same crystal plane (h k l) between simulation and test is fractional. Therefore, the simulated result by reflex is reliable. Only α-Al phase and Mg2Si phase are formed in theory shown by simulation. The calculated heats of formation and cohesive energies show that Mg2Si phase is easy to form and the alloying ability is more strong. Mg2Si phase is more stable than Mg17Al12 phase. Elastic modulus (E), shear modulus (G), bulk modulus (B) of Mg2Si phase is better than Mg17Al12 phase. However, the friability of Mg2Si phase is better and the plasticity is lower.