The invention relates to a preparing method for cured epoxy resin, in particular to a preparing method for low-dielectric and high-toughness cured epoxy resin. The preparing method aims at solving the problems that existing epoxy resin is high in dielectric constant and poor in toughness. The method comprises the steps that firstly, pentaerythritol and dimethylolpropionic acid are added in a reaction device three-opening bottle, p-toluenesulfonic acid is added, heating is carried out, the temperature is maintained for 0.5-1 h, heating and temperature maintaining are carried out again, then vacuum dewatering is carried out, a flexible chain end capping agent is added, the temperature is maintained, cooling is carried out, discharging is carried out, and flexible chain terminated hyperbranched polyester resin FCBHBP is obtained; secondly, epoxy resin, an anhydride curing agent and pyridine are mixed evenly, FCBHBP is added and stirred evenly, decompression defoaming is carried out, the mixture is poured into a mold, and curing is carried out. FCBHBP can improve the toughness of the epoxy resin through the double effects of free volume increasing and chain segment winding, and the impact strength and bending strength of the epoxy resin are remarkably improved; meanwhile, loss can be reduced within certain frequency through the interaction of flexible chains and polar functional groups in epoxy resin. The preparing method is applied to the filed of epoxy resin curing.