Water plays a crucial role in the synthesis mechanism of metal organic framework-5 (MOF-5). Synthesized MOF-5 with good phase structure and large specific surface area is largely determined by an important synthesis factor: the total water concentration of the initial synthesis solution (Ctw). An understanding of the effects of different and high Ctw on the synthesis of MOF-5 and the investigation of the maximum Ctw suitable for the synthesis of MOF-5 are important to guide the synthesis of MOF-5. Through the research of the maximum Ctw, a favorable synthetic approach was established which could realize the synthesis of MOF-5 with fine performance on CO2 adsorption and separation. The research results show that the maximum Ctw could be as high as 1,440mmol/L, and synthesized MOF-5 still has a good phase structure and a large specific surface area of 2,136m2/g (BET). Synthesized MOF-5 by the maximum Ctw exhibits a high CO2 adsorption capacity of 2.5mmol/g and a low N2 adsorption capacity of 0.2mmol/g at 298 K and 100 kPa. More importantly, synthesized MOF-5 by the maximum Ctw exhibits a high selectivity for CO2/N2 of 18-22 at 298 K and 20-130 kPa in simulated flue gas.