Recovery of waste heat from flue gas under 180℃ was named as deep recovery in this paper，and thermodynamics characteristics of this process were analyzed.Three recovery modes were proposed and some characteristics of them were also discussed.Especially，a mathematical model was built for split-flow pattern and the result obtained was discussed.There is obvious difference at the thermodynamics characteristics of recovery of waste heat in flue gas for deep and normal recovery，and exist of turn point Tco leads to a constriction for outlet temperature of hot water of single-flow pattern.Split-flow pattern can overcome this limit caused by the turn point，and split-flow ratio x can be determined from temperature of demand hot water and total cost.When cost ratio is higher than a low limit value，optimum split-flow ratio can be found to get the lowest total cost; while the cost ratio is lower than the value，split-flow ratio should be as small as possible to reduce total cost.When outlet water temperature of optimum result is lower than requirement，split-flow ratio should be improved or adjusted to reach demand value.