Abstract: Taking the developing trend of large-scale filled stopes in an underground mine as background，combining the urgent requirement of environmental protection in mines，taking many complex factors into full consideration，such as grain size composition，slurry viscosity，densities of slurry and carrier，material proportion，pipe diameter，roughness of pipe wall，installation quality of pipes，weighted average setting rate and so on，the critical conveying velocity inner pipes of different diameters was researched，based on the smallest resistance loss.The model of critical conveying velocity of full-tailings filling slurry of high concentration was established，and the affection laws of pipe diameter and concentration on critical conveying velocity were analyzed.The verification indicated that the calculation results of the model were reliable.In conclusion，the critical transport velocity regularly varies with pipe diameter，concentration and other factors.The results showed that with the increase of the diameter，the critical transport velocity of high concentration of the whole tailings backfill slurry increased by power function；with the concentration increasing，the critical transport velocity of high concentration of slurry tailing filling variation was reduced by cubic polynomial function.

Key words: Full-tailings filling slurry of high concentration, Critical conveying velocity, Filling pipe, Pipe diameter, Concentration