The invention discloses a compounded rubber reinforcing agent which is prepared from the following raw materials in parts by weight: 20 to 40 parts of garbage incineration fly ash, 1 to 4 parts of compound complexing agent, 30 to 40 parts of halloysite, 30 to 40 parts of activated carbon and 4 to 10 parts of water. Preparation of the compounded rubber reinforcing agent comprises the steps: (1) dissolving the compound complexing agent into water according to matching to form a compound complexing agent aqueous solution and adding the garbage incineration fly ash according to matching amount to obtain complexing garbage incineration fly ash; (2) drying and smashing the complexing garbage incineration fly ash, adding the halloysite and the activated carbon according to matching amount, putting into a high-speed mixing machine to be compounded and smashing after being compounded to obtain the compounded rubber reinforcing agent. The compounded rubber reinforcing agent can be matched with carbon black and white carbon black to be used, can improve wear resistance, water resistance, oil resistance and heat resistance of a rubber material and meanwhile achieves waste harmless and resourceful reutilization of the incineration fly ash. According to the compounded rubber reinforcing agent disclosed by the invention, a high-speed mixing and compounding technology is utilized, no wastewater or waste gas is generated in a reaction process, the technology is simple, and energy consumption is low.