Abstract: The mathematical model of complex forward flow multi-effect evaporation is established with condensed water to preheat feed directly, or with condensed water flash, with all or part of flash(vapor) used to heat feed, or all of flash vapor serves as the heating medium for the next evaporators. A calcu-lation method utilizing conventional iteration is presented. Utilizing object pascal, a calculation program corresponding to the above-mentioned method is developed. This program can be used to simulate two to seven effects ordinary and complex evaporation systems. Utilizing this program to calculate and compare five kinds of process of three effects forward flow evaporation process of sodium hydroxide solution is an example. The result of calculation shows that the model is reliable.