Abstract: Taking the transportation technology of ultrafine tailings cemented backfilling slurry with large diameter pipe as object，three dimensional model of large diameter transportation pipe was constructed.Based on reliable k-epsilon model of fluent-3D，the effects of slurry concentration within pipe on the transport properties of ultrafine tailings cemented backfilling slurry of were studied.The results showed that under the conditions that stowing ratio was 3 and the cement-sand ratio was 1∶6，the cavitation zone appeared at the inner pipe turning when cement backfilling slurry of ultrafine full-tailings was transported in 200 mm pipe.Along with the increase of concentration，the scale of cavitation enlarged at first and kept steady then；Along with the increase of concentration，the average velocity of slurry at inner pipe grew linearly if the concentration was less than 64%，and the trend of average velocity was increasing at first and decreasing then if the slurry concentration was greater than 64%；Along with the increase of concentration，the pressure loss increased with high speed if slurry concentration was less than 68%，and the pressure loss grew slowly if slurry concentration was greater than 68%.It is pointed out that the best transportation concentration with 200 mm pipe for cemented backfilling slurry of ultrafine full-tailings was 66%.

Key words: Large diameter pipe, Slurry concentration, The scale of cavitation, Average velocity, Pressure loss