An anti-settling micro-nano composite insulating material and a preparing method thereof are disclosed. The material is prepared by adding a micro-nano filler subjected to surface modification treatment into an epoxy resin matrix and an anti-settling agent, then adding a curing agent into the mixture, performing dispersion treatment, degassing the mixture at 30-40 DEG C, heating the mixture to 50-60 DEG C, pouring the matrix into a mold, curing, cooling and demolding. Through the anti-settling agent and the micro-nano filler subjected to surface treatment utilizing a silane coupling agent, dispersity of filler particles in the epoxy resin matrix is improved, and the filler particles are not liable to settle in an epoxy resin composite material manufacturing process, thus improving properties of the micro-nano composite insulating material, preparing the anti-settling micro-nano composite insulating material and providing technical guarantees for subsequent research of the micro-nano composite insulating material.