The invention relates to pressureless sintered conductive silver paste which comprises: 70 -85% of silver powder, 5 -20% of solvent, 0.1 -2% of a dispersing agent and 0.5 -5% of an organic carrier, wherein the silver powder is composed of micron silver powder and nano silver powder. The micro-silver powder is subjected to surface modification, and silver nano-clusters are formed on the surfaces. The characteristics of low-temperature sintering of the nano-clusters are utilized, the sintering of the nano-particles and the connection of the micron-silver powder are promoted, and the low-temperature sintering compactness of the conductive silver paste is improved. The bonding and interconnection of a chip and a metal layer on the surface of a substrate are realized. The conductive silver paste prepared by the preparation method disclosed by the invention has the characteristics of high temperature resistance, high heat conductivity and high bonding property, and the reliability of the packaging device can be remarkably improved. The pressureless sintered conductive silver paste and the preparation method thereof are suitable for bonding and heat dissipation of a third-generation wide-gap semiconductor chip.