Abstract: The essential requirements for the ultrasonic automatic inspection equipment specified in Shell Standard DEP 31.40.20.37-Gen—2011 Line Pipe for Critical Service are described. The arrangement of each probe group and their working principle and the selection method of artificial defects on a reference sample pipe for detecting defects in all zones with different wall thickness of 30%t, 50%t, 70%t on a longitudinal seam submerged-arc welded pipe in the on-line ultrasonic automatic inspection system are proposed. With many detected cases, some targeted experiments and explorations are conducted for the selection and arrangement of probes, which results in effectively ensuring that 100% inspection of all zones of the inner weld seam within the range of the wall thickness of 31.9 mm can be covered by using four groups of longitudinal probes and two groups of tandem probes so as to increase the inspection capacity of the on-line ultrasonic automatic inspection system and improve the work efficiency of the ultrasonic automatic inspection and consequently to insure the pipe quality.