Belonging to the field of adding devices, the invention provides a composite modified asphalt admixture adding device and an adding method thereof. The device comprises a high-speed stirring apparatus, a composite reaction apparatus, admixture containers, admixture control valves, a high-speed stirring apparatus support, and thermal insulation and heating apparatus. The high-speed stirring apparatus is placed on the high-speed stirring apparatus support; the composite reaction apparatus is used for placement of asphalt and serves as a composite reaction container; and the thermal insulation and heating apparatus is used for maintaining the temperature of the composite reaction apparatus. The device provided by the invention has the advantages of simple operation, safety and reliability, can greatly reduce the requirements for operators, also can effectively reduce the workload of operators, has no need for stopping stirring and other operations in an admixture adding process, also can enable uniform diffusion of the admixture into asphalt, and reduces stirring time. The test result of the invention is true and reliable.