The invention discloses a preparation method for a high-density interposer for microelectronic system-in-package. The preparation method comprises the following steps of: 1, preparing a drectionally growing carbon nanotube bundle array, wherein the diameter of each carbon nanotube bundle is 0.5 to 30 microns, the gap of the carbon nanotube bundle is 0.8 to 100 microns, and the carbon nanotube bundle is 40 to 500 microns long; 2, depositing metal tungsten on the surface of the drectionally growing carbon nanotube bundle so as to form a conductor array; 3, melting borosilicate glass and compounding the melted borosilicate glass with the conductor array so as to form a compound body; and 4, grinding the upper surface and the lower surface of the formed compound body so as to expose the end of the carbon nanotube bundle deposited with the metal tungsten, and obtaining the high-density interposer for the system-in-package. The material adopted by the preparation method has low thermal expansivity and the process method is low in time consumption, so the prepared high-density interposer has the advantages of high density, high reliability and low cost.