For engineering geological drilling operations at sea, mud protection is necessary in sand, loose rock and weathered rock stratum, and the best effects can only be achieved by using the mud prepared with fresh water. By establishing a two-process model of mud making with seawater, primary pure water produced by the first-stage process device can meet the water standards for high-performance mud; ultrapure water suitable for drinking is output by the second-stage process with the residual salts being removed. The designed seawater mud making device, including seawater pretreatment and reverse osmosis devices, EDI separator and maintenance control system, breaks through the the restriction of platform space, load, fresh water cost and other factors in the offshore exploration, the exploration cost is effectively reduced.