Based on the principle of the 'mise-a-la-masse' method and the result of model pile testing, this paper deals with electric field characteristics around the reinforcement cage. Tests reveal that the bottom of the reinforcement cage corresponds to the minimum of the electric potential gradient curve and the inflexion of the electric potential curve. A method for reinforcement cage length detection in the core drilling hole is accordingly presented. Practice shows that the method is convenient, practicable and credible.