The model of complex cocurrent multi-effect evaporation with condensed water flash and extra vapor elicitation is established. The conventional model of cocurrent multi-effect evaporation is a simplified example of the model. A new algorithm (iteration method combining with matrix method) is presented. The results show that the new algorithm is which increases the convergent speed and stability, more efficient than the Newton method and Broyden method. With condensed water flash and preheating cane sugar solution from 26.7 to 90℃ by elicitation extra vapor,the fresh vapor expenditure of cocurrent three-effect evaporation is decreased by about 16%.