Abstract: As an essential tool for pipeline inline inspection, the caliber detector plays an irreplaceable role in eliminating the hazards of pipeline. Based on the principle of the caliber detector, the swing of detection arm when the detector passes the tee section is analyzed. The dip-angle changes of the detection arm close to the bypass section are obtained – the closer to the center line of the bypass pipe, the more greatly the dip-angle changes (to the limit of 0o). Moreover, the biggest fan area when the detection arm angle is 0o is obtained. The number and probability that the detection arms of the detectors with different structures stay in the fan area are defined. According to the quantity when the detection arm angle is 0o, a new tee recognition algorithm is put forward, which can effectively avoid false detection and improve the detection precision of the caliber detector. The research results provide significant reference for research and development of caliber detector technology.