The effect of non-isothermal mixing on system energy target in heat integrated water networks was addressed，and the concept of “split temperature” was proposed.For homogeneous mixing，system utility requirement will not decrease.For heterogeneous mixing，it can identify whether system utility requirement would increase，decrease or keep the same，by locating the split temperature and comparing the cumulative heat of the mixing streams above the split temperature and the heat demand in the lower temperature intervals.Based on the ideas above，some rules of non-isothermal mixing are presented，which could be used to estimate whether energy penalty exists or not，and to eliminate the penalty by designing new mixing temperatures.Combining them with improved “separate system”，a systematic method is proposed to design energy efficient water networks.