The invention discloses a special admixture for a self-compacting and non-shrinkable concrete filled steel tube. The admixture comprises the following components by weight: 30-50% of light roasted calcium oxide clinker, 10-30% of light roasted magnesium oxide clinker, 10-30% of plaster, 10-30% of flyash, 1-3% of a polycarboxylic acid high-performance water reducing agent, 0.1-1.0% of a thixotropic agent, 0.2-2.0% of a plastic foaming agent, 0.2-1.0% of a retarder and 0.5-2.0% of a defoamer. The special admixture has the beneficial effects that the mobility of newly blended concrete is improved, and compact filling of a mixture in the steel tube is guaranteed; the thixotropy of the newly blended concrete is enhanced, after a corresponding part is filled with the mixture, gel strength rapidly recovers and is improved, the movement resistance of entrapped bubbles is greatly improved, therefore, the entrapped bubbles cannot float or gather, and the top separation of the steel tube is remarkably improved; and the plastic foaming agent and a calcium and magnesium composite swelling agent completely compensates shrinking of the concrete filled steel tube in a whole process at different stages, therefore, the concrete is compactly attached to the wall of the steel tube, and cooperate stress is implemented.