This study deals with the transient response of heat exchanger which is approximated to the first-order system with delay. This function is of simple form and easy to be applied to design the practical controllers. The parameters such as gain K, time constant T and delay time are derived in this paper. On the basis of using the first-order system as the transfer function for a heat exchanger, more accurate models are built to derive the parameters K, T and respectively, which are constructed with a lumped parameter approach. Unlike a number of recent analyses K is obtained by using arithmetic mean temperature difference, log mean temperature difference has been employed instead, which minimized the error; time constant T is obtained by deriving the time constant of baffle; delay time is estimated by flow rate. The result shows that this method to study the dynamic characteristics of heat exchanger is easier, more accurate.