Abstract: Shell-and-tube heat exchangers with helical baffles are now widely used in many engineering fields，especially in chemical and petroleum refining industries.The helical lead distance is one of the important design parameters.The continuous four-sector baffle was taken as an example,and the theoretical calculation method of the helical lead distance of the shell-and-tube heat exchanger with helical baffles was deduced.Then this procedure was extended to continuous n sectors baffle，continuous baffle and staggered sectors baffle.Finally，an equation to determine the helical lead distance for any kinds of helical baffle was proposed.A short discussion of the determination methods proposed in the literatures was presented.