Abstract: Multi-effect multi-stage flash process is one of the evaporation processes in chemical engineering, yet it is somewhat different from conventional multi-effect evaporation. To meet the new problems encountered methods of analysis and formulas for calculation are presented here from the theoretical aspect. First, the multi-stage flash is analyzed in order to get the stagewise changes in flash variables such as flow rate, concentration, pressure etc. Schemes of circulation of concentrated solution in each effect are then analyzed for the determination of circulation parameters and terminal variables in each effect. The peculiarities such as double-streams, multi-stages and combination of flash and condensation in a section are then analyzed, stagewise changes in source and sink temperatures are determined, and various solutions both for design and operation problems are presented. The results thus obtained have been checked with the published data of a desalination unit and they agree well. It is suggested that these methods of analysis are applicable to all flash vapourization technologies.