Abstract: A sensitivity theory was introduced to study the operation properties and regulated characteristics of heat exchanger networks，a flexibility identification method based on sensitivity index was established，with which the flexibility space and flexibility properties can be obtained. The sensitive fluids in the heat exchanger were identified through sensitivity analysis，and a by-pass adjustment method was introduced for the control of these fluids. Hence，the fundamental rules and methods of by-pass adjustment of the heat exchanger and its networks were established. An application instance illustrated that the by-pass can reduce the disturbance transmission in the fluids which alters frequently and enables the network working in harsh conditions. The by-pass can guarantee the stability of most outlet parameters of fluids，which can reduce the number of utilities without cost increase. Meanwhile，the operation and structure optimization was also achieved by the by-pass adjustment.