Detecting coal mined-out areas with effective measures has been major issue to be addressed in mining area environment management, mine safe production and major construction projects site selection. Based on the fundamental principle of frequency domain electromagnetic method (FDEM), method characteristics of EH-4 and CSAMT was compared and analyzed, electrical characteristic of mined-out area filled with different materials was analyzed through numerical modeling, and practical detecting effect was discussed by typical project cases of EH-4 and CSAMT. Numerical modeling result and engineering practice prove that compared with surrounding rock mined-out area has prominent electrical difference, and base on this physical property mined-out area detecting with FDEM can obtain ideal effect. FDEM is an effective method in detecting coal mined-out area and it has already been an effective technological means in coal mine water prevention and control and geological disaster survey of mined-out area.