The mechanism of liquid phase back-mixing in the liquid-solid fluidized bed has been studied.The radial parabolic distribution of liquid velocity is found to be the main motivation of particlescirculating movement which in trun causes the liquid back-mixing.The liquid back-mixing can be reduced by evening the radial distribution of liquid velocity.This is achieved by a hetero-pores distributor which has a resistant coefficient distribution analogous to that of liquid velocity.The experimental result shows that the hetero-pores distributor reduces the liquid back- mixing effectively.