Based on an analysis of magnetic anomaly characteristics inside the perpendicular cylinder and the result of model piles test, the authors studied the magnetic anomaly characteristics inside the reinforcement cage of the cast-in-situ pile. The results indicate that the magnetic anomaly curve along the borehole depth inside the reinforcement cage has the form "a valley between two peaks, on which a wide negative anomaly region like a saddle corresponds to the middle part of reinforcement cage". The bottom of the reinforcement cage corresponds both to the inflexion of the magnetic anomaly curve and to the maximum of the magnetic anomaly gradient curve.