Abstract: The mine tailings generated from metallic ore not only occupies a large area of surface resources but also easily causes mud-rock flow and tailings dam failure. Moreover, the existence of a large number of underground voids threatens the safety of underground mining operations and can induce mines earthquake and surface subsidence. The paste filling technology involves thickening the mine tailings into paste and placing the paste in underground voids. The technology has been widely accepted and applied around the world for its advantages in safety, environmental protection, economy, and high efficiency. The dewatering of mine tailings is a prerequisite for the paste filling process. In the paste backfill, after tailings thickening, the concentration increase is limited, the yield stress is increased, and fluidity is reduced with flocculant dosage. The flocculant dosage and thickening synergist work together to further increase unclassified tailings paste concentration and reduce slurry yield stress. The mechanism of the thickening synergist was researched from a microscopic point of view. The results show that the best addition method is to add thickening synergist after tailings settlement with flocculant dosage by settlement and rheological test. The solid mass fraction can be increased by 8.57%‒10.13%, and the yield stress can be reduced by 6.68‒12.85 Pa. The multi-component thickening synergist can not only reduce unit consumption and cost but also improve the compressive strength of the paste backfill material. The compressive strength of paste backfill material with thickening synergist and cement-tailings mass ratio of 1∶12 is 2.5 MPa at the age of 28 d. The difference is less than 20% compared with the compressive strength of the material with cement-tailings mass ratio of 1∶6 and without thickening synergist. By total organic carbon adsorption test and Zeta potential test, the synergist is found to have functions of adsorption and dispersion. It can destroy the flocculation structure and release the contained water, thereby increasing the tailings concentration and improving the fluidity of tailings particles.