Aimed at suitable size of farming unit in the system of Precision Agriculture, this paper presents a detecting method in two directions to get the vertical & horizontal indexes of the multidimensional spatial variability of soil chara, crop growing status and the previous yield, and then to determine the size of farming unit adaptively, according to the agronomy principles. The result show that the fertilization equilibratory effect of favorable rate is greater than 90%, when we use the variable rate of farming unit size determined by this method. Another characteristic of this arithmetic is that it is easily applicable for both of vector and grid data format of GIS.