Abstract: Aimed at the question of material ratio of cement paste backfilling (CPB)，through analyzing the material composition of the internal structure in CPB，it was proposed that the larger aggregate packing degree is，the better paste flow；The Fuller theory was used to analyze the filling material gradation characteristics of a certain copper mine，and explore the rule of grading index n changing with material ratio.The slump test and rheological experiments of CPB were carried out，and Buckingham formula was adopted to calculate the pipe flow resistance.The results showed that the aggregate packing degree is larger and is most favorable for pipeline transport，when the material ratio with unclassified tailings of water quenching slag as 3of 2.The pipe flow resistance was 2.46 MPa/km under the conditions of mass fraction 72%，pipe diameter of 150 mm，flow rate of 60m³/h and water quenching slag content 40%.By contrast with the pipe flow resistance of unclassified tailings 3.65 MPa/km，its drag reduction efficiency reached 32.6% in pipeline transport.That illustrated that the paste pipe flow resistance is reduced obviously after gradation optimization.

Key words: Paste backfilling, Material gradation, Aggregate packing degree, Fuller grading curve, Pipe flow resistance