The ratio of outlet temperature of hot fluid to that of cold one,a, which reflects the heat transfer depth, greatly influences the effective heat-transfer temperature difference. In order to investigate the relationship between the length-to-diameter ratio and the heat transfer depth of shell-and-tube heat exchangers, the heat transfer performance in the baffling flow region in the shell side is numerically analyzed via a flow path approach and is compared with that in the counterflow region. The results show that （ 1 ） at the critical point （ a = 1 ） of heat transfer, the heat transfer performance in the baffling flow region is far poorer than that in the eounterflow region, so that the baffling flow region should be kept far from the critical point; （2） as the deviation of heat-transfer temperature difference between the baffling flow region and the counterflow region Changes with a, the ratio of baffling flow area to the whole area should be controlled to prevent the deviation from being too large ; and （3） when a 〈 1, the ratio should be smaller than 0.6/R1a,c to keep the deviation within 5%. The reasons for the difficulty m achieving an a vatue less than 1 for large-scale heat exehangers are also revealed, and a multi-parallel ehannel strueture in the shell side is finally presented to inerease the heat transfer depth.