Abstract: ‘BT’ method is a method in application of limited unit method to dredging the trunk line of underground water in the mineral area and supplying water for urban districts, factories and mines. The procedure is, first to determine the water level value (BT value) changing with the passage of time at each joint on the border (p1) by applying analytics, and then to calculate the inner joint and the water levelv alue at the joint on the border (p2) by using limited unit method; hence all the work, from parametric optimization to predication of water level at the inner joint, are done automatically by computers.