Abstarct: Due to population growth and technology advancement and according to limited and exuberant nature of fresh water sources, some areas of the planet earth especially developing countries will suffer from a depression crisis. given that the oceans and seas are massive water resources on the planet earth, therefore, the use of sweeteners to sweeten the waters of the seas and oceans can be a good option to deal with this crisis.Common thermal water desalination like MSF and MED, are appropriate for height capacities of water production, and are not designed for the local usages. But due to the development of distributed power generation in the country it is also necessary to develop a suitable method for low capacities. HD desalination system is one of relatively new methods for dispersing water production in low capacities. HD system is made up from three parts: humidifier, dehumidifier and heat source. The heat source of such a systems could be solar or low temperature heat lost sources. So in this research a closed air cycle HD desalination system which drives with 1-MW natural gas engine heat loses is modeled and designed. To do this, first, engine part equations and two main parts of desalination system: humidifier and dehumidifier equations are extracted and then the full model is provide in the Matlab environment. Considering the salty water properties on fresh water production in modeling, Investigating the effect of closing the air cycle relative to open cycle in the past work, Evaluate the effect of pressure change in each of the two main parts of the sweetener and Modeling the engine and examining the effect of the heat exchenger pressure drop on power generation Among the cases discussed in this thesis. in the following, a parametric analysis and the performance of desalination system were studied. The results of the parametric analysis shows that by increasing salinity the amount of production for both open and closed cycles decreases, With the difference that the production value for the closed cycle is 5% more than the amount of production for an open cycle. Also by reducing the pressure of the humidifier and increasing the pressure of the dehumidifier, Compared to fixed pressure system (under atmospheric pressure), for humidifier pressure of 50-kpa and the pressure ratio of 2.3 production increases by 60%. Finally the design of proposed system is baxsed on the parameters obtained for both constant and variable system is presented. For the proposed scheme, by a 1-MW engine and using a fixed pressure system about 17.8 cubic meters per day can be produced, which this amount for a variable pressure system increased to about 20.6 cubic meters per day.