The invention provides a diverting agent having both instant blocking and supporting effects and a preparation method thereof. The diverting agent contains a macromolecule degradable shell and a high-compressive strength inner core, wherein, the macromolecule degradable shell and the high-compressive strength inner core are adhered by an adhesive; the macromolecule degradable shell is formed by fusing and mixing two or three of polylactic acid-glycollic acid, polylactic acid and polycaprolactone; the high-compressive strength inner core is one of ceramsite and silica sand. In the process of instant blocking, diverting and crushing, the diverting agent is in a gap, macromolecules can degrade the shell due to a stratum extrusion effect to generate plastic deformation to form an effective instant blocking belt and ensure instant blocking effect. After construction is completed, the macromolecules can degrade the shell and the shell generates self degradation under the effect of a formation temperature, high-strength inner core is remained in the original place to support cracks and maintain the aperture of the crack, thereby effectively solving the problems that the crack is closed due to complete degradation of the instant blocking agent in the process of instant blocking, diverting and crushing and productivity is reduced.