Electrostatic detection devices can detect humans by measuring changes in an electrostatic field to indicate the precise location and direction of the subject entity. To analyse the principle of the electrostatic detection system, the mechanisms for generating the human beings' bioelectrical phenomenon are analysed. The human being is regarded as a point charge and constitutes a system with the obstacle and the air. Based on the principle of static induction, the human is detected through the obstacle. The distribution of the human body's electric field is studied using the image method. And the mathematical formula of the point charge's electric potential in this system is developed. Accordingly, the mathematical model of this human detection system through the medium is established and tested by using MATLAB. And the detection principle of electrostatic detector through medium is introduced. It is concluded that the thickness and dielectric constant of the medium are in an inverse relationship with the goal's electric-field intensity through the medium. It is proved by the experiment that it is feasible using electrostatic detector to detect human beings through the medium.