Radicand number is converted to [0.25, 1) within the range of the linear interpolation calculation open square approximation result; Under the accuracy not being deteriorated case, [0.25, 1) is divided into [0.25, 0.5) and [0.5, 1) two sections, respectively using single and double interval constructed interpolation nodes, so the interpolation node data table reduced by one third; bringing interpolation error into the value of the node, not to increase the additional amount of computation so that the the interpolated absolute error decreased half . the paper gives one multiplication, two plus / subtraction, and a small amount of shift operations to gain the implement methods of the square root.