Abstract: Abstract The main factors for affecting the fluidity of filling material are analyzed under the slag cementitious system without clinker，The fluidity tests results show that the addition of fly ash can significantly improve the liquidity of the system. The slurry fluidity increases with the increase of fly ash content within a certain range. With the fly ash content of 15%，the slurry fluidity reaches maximum 220 mm; The slag has less improvement on the slurry liquidity effect than fly ash，but can improve the compressive strength of the filling body at each age; The tailings at appropriate grade have positive effect on liquidity with the increase of maximum 46.7%; The slurry concentration has a great impact on the liquidity and strength of the system. The slurry with concentration of 73% has a good workability and its fluidity is up to 240 mm maximum.

Key words: Steel slag, Cemented backfilling, Fluidity, Filling strength