The invention relates to a preparation method of a modified compound filler filled breakdown-resisting epoxy composite. The preparation method comprises the following steps: through the modification of a silane coupling agent gamma-APS, amino groups are introduced on the surfaces of Al2O3 and h-BN; hyperwheworries branched polymers (HBP) are grafted on the surfaces of the primarily modified fillers through taking the grafted amino groups as active sites to obtain a modified filler Al2O3-HBP and a modified filler BN-HBP; the two modified fillers are fully mixed with an epoxy resin matrix in different proportions; the epoxy composite is prepared through a two-step stepped heating and curing method. The alternating-current breakdown strength of the prepared epoxy composite shows the obvious cooperative behavior, the system breakdown strength can be regulated through changing the proportion of the fillers; when the BN-HBP accounts for 80 wt% of the compound filler, the performance of the BN-HBP is optimized, only 10 wt% of compound filler is added, and the alternating current strength of the epoxy composite can be improved by 15.7% as compared with that of the epoxy matrix.