The invention belongs to the building field, and concretely relates to a high efficiency composite expanding agent for a high-strength self-stressing concrete filled steel tube and a preparation method thereof. The high efficiency composite expanding agent comprises the following components by mass percentage: 22-40% of light calcined magnesia, 12-25% of active calcium oxide, 6-14% of calcined dolomite, 18-30% of sulphoaluminate cement clinker, 12-18% of natural gypsum and 2-6% of modified meerschaum powder. Through a design of hydration expansion characteristics of different components, expansion performance and high-strength concrete intensity are cooperated for development, expansion usefulness is high, water for hydration is little, expansion beginning time is early, and the expansion performance is stable, by introducing a maintenance material, problems of difficult maintenance of a steel pipe under enclosing condition and difficult supply of expanding agent and water for concrete hydration can be solved, and the high efficiency composite expanding agent is suitable for preparing the high-strength self-stressing concrete filled steel tube under enclosing condition of the steel pipe.