Mass fraction and waste rock proportion are main influence factors which affect flow characteristics of backfill paste mixed by unclassified tailings and waste rock. To study of the influence law of these factors about paste flow characteristics may have important significance about controlling paste flow characteristics. Shear test of mixed backfilling is conducted with the improved small direct shear device and then get the change law of mixed backfilling. The test results show that paste presents approximate elastic at prophase of force-deformation and the stepladder trend in displacement-shear stress curve increases with the increase of waste rock proportion. Cohesion of paste depends on mass fraction and internal fraction angle is affected by mass fraction and waste rock proportion almost the same

and that is combined effect. When mass fraction in the range of 80%-88%

cohesion pretends as nonlinear growth with the increase of mass fraction

internal fraction angle and mass fraction presents S-type trend

shear strength increases with the increase of mass fraction and varies significantly when in the range of 82%-84%; at the same time

flow characteristics is mutation.