The invention discloses an epoxy nanocomposite based on polyhedral oligomeric silsesquioxanes (POSS) and a preparation method of the epoxy nanocomposite based on the POSS and relates to the epoxy nanocomposite with a controllable phase structure capable of improving effect and based on the POSS. The preparation method includes the following steps of allocating a POSS monomer, a chain transfer agent and an initiating agent in a solution according to the measuring amount through a reversible addition-fragmentation chain transfer (RAFT) method, conducting freeze thaw and degassing through liquid nitrogen, charging argon shield to enable the mixture to react, and obtaining PMAiBuPOSS; further allocating PMAiBuPOSS, Methl Methacrylate monomer (MMA) and an initiating agent in a solution, and obtaining POSS base block copolymer , namely, PMAiBuPOSS-b-PMMA, through the RAFT; conducting melt blending on the POSS monomer, epoxy resin, the prepared POSS base block copolymer to obtain blend precursor of the POSS and the epoxy, adding in a curing agent, fully stirring, dissolving the mixture the curing agent, pouring the blend into a die, conducting temperature programming and curing, and obtaining the nanocomposite with the controllable phase structure and based on the POSS.