Abstract: Generally，the concentration of solutions in inorganic salt industry all adopts multi-effect evaporation method.The steam consumption can be saved by multi-effect evaporation system，but equipment investment will be raised as the increasing of the effect number.The effect number is limited by equipment investment，depreciation cost，and heat transfer temperature difference，so the infinite increase of effect is impossible.When the equipment investment is more than the energy saving，adding effect number is meaningless.The question of selecting the best effect number was very important.Because there are many variables，the selection was very difficult.Correlation between the best effect number and annual minimum cost of consumption was linked, a function for annual cost goal was established and a simple formula for the best effect number was proposed.According to the actual engineering，the formula was simple，feasible，and reliable.

Key words: multi-effect, evaporation, the best effect number, annual minimum cost of consumption