Abstract: Different influent flow rates of 150-500 L/h and different sludge heights of 0-45 mm were combined in anaerobic baffled reactor (ABR).The liquid phase flow characteristics in the first compartment of ABR were investigated by laser particle image velocimetry (PIV), and the liquid phase flow field of the critical section of the reactor obtained. The results showed that when the sludge height was 0 mm, the radial mean velocity of the ABR down-flow zone decreased first, then increased and finally decreased with the increase of inflow rate; when the sludge height was 15 mm, the axial mean velocity decreased first and then increased with the increase of inflow. The vorticity intensity increased gradually with the water inflow, and the radial and axial mean velocities fluctuated upward and downward with the increase of inflow. With the increase of sludge, the degree of back flow in the down-flow zone became larger. The high vector velocity area was concentrated on the 180-250 mm away from the inlet of the down-flow zone. The number of vortex cores increased with the increase of sludge height, and the high vector velocity area was concentrated on the lower part of 0-20 mm.