Abstract: Four rules of retrofit of heat exchanger networks (HENs) are proposed through qualitative analysis of HENs：①The controlled variables (CVs)are sensitive to the manipulated variables (MVs)；②The number of the selected MVs should be less than or equal to the number of the CVs；③A set of the controlled variables possess a set of the manipulated variables, and the changes in the CVs induced by the changes of disturbance variables (DVs) can be adjusted and their increments are equal to zero at the end by the adjustment of the selected MVs；④If the existing HEN is optimal in normal operating condition, and the minimum change of the existing HEN during retrofit makes the HEN suitable for present or multi-period operating conditions, then the retrofit cost may [HJ]be the minimum, so the retrofitted HEN may be optimal. The mathematical models and solving method of retrofit and control of HENs are established based on the rules and sensitivity analysis of HENs. The procedure of the retrofit of HENs is given. It is established by the calculation of the examples and practical use that the method is correct and available.