Abstract: The system using personnel - computer to plot field strength maps automatically has been proved to be effective for practical purpose . It firstly sets up a database including the terrain and the occupied area ratio of the building . Once the parameters of trasmitting and receving ends are inputted to the computer , the field strength map of the area required to predict can be plotted automatically. Thus the efficiency , precision and reliability can be improved greatly . This paper shows a series of expressions used in the prediction of field strength at the same time .