With the fast development of the economy and huge amount of population, fresh water shortages have become very serious in China, especially in northern coastal cities where China's population and economy are concentrated. The coal-fired power plant integrated with low-temperature multi-effect distillation (LT-MED) is a viable way to solve or eliminate the problems in China as the coal-fired power plant dominates in power industry and LT-MED has technical advantages in using low temperature heat resources over other desalination technologies. An accurate assessment on energy requirement of water production is of great importance in dual-purpose plants since the cost of energy requirement accounts for the foremost part of the water production cost. This paper presents an economic analysis of the energy cost of water production for dual-purpose plants by the equivalent enthalpy drop theory (EEDT) and the off-design analytical theory on steam turbines. EEDT that is proven to be an effective method for partial quantitative analysis can be used to accurately calculate both the energy requirement of water production and the economic influence of extraction steam on steam turbines. By solving the matrix models of EEDT, electricity-equivalent consumption rate (EECR) for water production and reduced steam turbine efficiency ratio were analyzed at off-design conditions for LT-MED combined with thermal vapor compression process (TVC) process. The results indicate that EECR is more accurate to be used to evaluate the economics of the desalination process than GOR that is widely used. They also show that EECR that is the function of the equivalent enthalpy drop, the share of the extraction for heating the desalination process, and GOR reflects the real work ability of extraction. Besides, it is seen that the specific fuel cost for water production is low compared with that of reverse osmosis. It is concluded that the coal-fired power plant integrated with LT-MED is an economical and suitable way to resolve the water shortages in northern coastal zone of China.