Abstract: This paper first time makes a proposition about detection of an evilcave existing in soil structures such as embankments and railroad-foundations. According to the characteristics of distortional electric field, the measurement and an ideal model are established. In idealized soil body which has constant conductivity and a globular cave inside, the distributive characteristic of spatial electric field is found by solving Laplace equation 2U=0 which is a boundary valure problem. A measurable criterion on surface electrical potential of the soil body is established. An imagined numberal example indicates that an evil-cave of size about 0.2 m in depth 4.0 m from surface of the soil body can be measured. It will be able to give a judgment in field if setup an electronic computer.