The invention discloses Mg-Ca-Al alloy high in Ca/Al ratio and a preparation method. The alloy comprises, by weight, 4-12% of Ca, 5-10% of Al and the balance of Mg or a solid solution of Mg, and the mass ratio of Ca/Al is 0.8 to 1.2. The alloy is cast through a direct water cooling semicontinuous casting die or a rapid cooling casting die, microscopic structures comprising net-shaped Al2Ca structures are obtained, the net-shaped Al2Ca structures are fully smashed into micrometer or submicron particles through extrusion, and meanwhile, crystal grains are further fined so that a Mg-Al2Ca composite material can be obtained. The prepared Mg-Ca-Al alloy has good mechanical property, high burning temperature, high anti-corrosion performance and superelasticity, and the defects that existing Mg alloy is prone to corrosion and combustion and difficult to machine thermally, and application of the Mg alloy is seriously affected are overcome. The alloy solves the problems that the existing Mg alloy is prone to corrosion and combustion and difficult to machine thermally to a certain degree and can effectively expand the industrial application value of the Mg alloy.