The invention relates to an adhesive used for a high-heat resistance leadless low-dielectric copper-clad plate and a preparation method thereof. The adhesive comprises the following components in parts by weight: 15-70 parts of low dielectric epoxy resin, 15-40 parts of modified polymerized acid anhydride, 5-40 parts of a cyanate esters curing agent, 2-40 parts of an active esters curing agent, 10-50 parts of a bromine-containing fire retardant, 0.05-1.000 parts of silane, 0.001-0.1 parts of a metals curing accelerator, 0.001-0.1 parts of an amines promoter, 0.005-0.1 parts of an imidazoles promoter, 10-60 parts of an inorganic filling material, and 1-8 parts of a flexibilizer. Compared with the prior art, the copper-clad plate has the advantages of high heat resistance, low water absorption, low dielectric loss and excellent resistance to heat and humidity.