The thickness and refractive index of Zn1-xMgxO film grown on A-sapphire substrate by molecular beam epitaxy were measured by ellipsometry. Combined with Mg content measured by inductively coupled plasma (ICP), the curves showing the relationships of thickness with film growth condition and the refractive index with the Mg content in the film were deduced by numerical analysis, which may serve as a theoretical basis for controlling the thickness and the refractive index in Zn1-xMgxO film growth process.