Abstract: In order to get an effective perspective width for a coal face,a field strength formula of radio wave penetra- tion for coal face is derived,which is suitable for measuring field strength values observed from receivers. The field strength H is determined by initial field strength H0 ,transmit-receive distance r and absorption coefficient β. To the same H0 ,r is controlled by β,the smaller β can get a farther distance and be more suitable for detection. A new method to calculate H0 and β values is put forward,the H0 values can be derived from the H data near the transmitter in the transmitting gateway,while β values can be derived from the H0 values and H data observed in the receiving gateway. The calculated H0 and β values are more reliable than those derived from single gateway,transmitting or receiving gate- way. It is also put forward that the effective perspective width value for coal face can be calculated by the H0 ,β and background field strength HB with a special instrument and operating frequency. The tests in the coal faces of Xieqiao Mine,Huainan Coal Mining Group Corporation,demonstrate that the calculated initial field strength values and absorp- tion coefficient values are more objective,and the effective perspective width values determined by H0 ,β and HB are more reliable,which can be used to guide the design and detection of radio wave penetration for coal faces.