Abstract: The paper introduced working principle of the high density resistivity method and the theory of 3D finite-difference forward modeling of resistivity in details. In view of characteristics of karst in coal mine underground, it established a specific karst model,and synthesized reverberation profiles by forward modeling. The results of calculation showed that using 3D finite-difference of high density resistivity method to forward model of karst of coal mine could observe features of the karst and this can provide theoretical basis for treamenting hidden karst of coal mine.