This paper designed a new device for producing fresh water. In order to solve the problem of freshwater shortage in the offshore drilling platform or barren island, Firstly the device will change wave energy into gravitational potential energy of the floater. Then the gravitational potential energy of the floater is converted into mechanical energy by the intermediate transmission device. and mechanical energy is converted into pressure energy by the vacuum pump. The pressure of the system is reduced. The saturation temperature of water is further fall. Then the evaporation of seawater required heat is reduced. so the device has the effect of saving energy. Firstly, this paper established the mathematical model in the steady state. Through the analysis of the mathematical model, water production can meet the fresh water requirements of offshore drilling platforms, the device has the economic efficiency and good energy saving effect.. Research contents Firstly, through consulting data and sampling methods, we determine the amount of fresh water of offshore drilling platform on one day, and the minimum water production capacity should be determined; and determine the structural frame work platform sea area near the stable system and weather related natural factors (example: wind speed, wave height, wave cycle).The related natural factors in each season the working platform nearby the offshore drilling platform should be determined.(example: wind speed, wave height, wave cycle). Secondly, according to the analysis of survey results and data, we calculate the dilution tank capacity and the minimum pressure to bear; designing shape of floater , the determination of its size, the reasonable selection of materials, rational design of the wave shape, determine the size, screening materials; reasonable arrangements for the design of negative pressure fresh water conversion device installation. The reasonable installation of the device on the drilling platform Thirdly, we consider the low limit, tank capacity and the size of floater, and errors should be taken into account, then we can determine the working volume of vacuum pump, selecting materials, reasonable pipeline design; determining the size of solar panels and the calculation of the generation. Fourthly, we calculate the minimum driving force, the average working distance of float and power of the pump by the data Fifthly, we considering the various data, then testing weather design of the device is very reasonable through the reasonable calculation; And according to the minimum driving force and float force, we calculate parameters of intermediate transmission mechanism, the design efficient transmission device; by considering the proper placement of solar energy device. Fig. 1 is the research process. Fig. 1 the research process