The 25 Hz phase-sensitive track circuit faces a broken rail detection problem due to the presence of a bypass path. As a result, the variation law of the receiving end voltage under broken rail conditions has not been clarified in the field operation for a long time. To provide a theoretical basis for eliminating potential safety hazards, based on the multi-conductor transmission line (MTL) modeling method, multiple sections along the bypass path of a 25 Hz phase-sensitive track circuit are equivalently represented as a single bypass section. A six-port network is adopted to analyze the voltage and current relationships between the broken rail section and the bypass section. These sections are linked through the impedance bond (IB) and the earth to form a coupling circuit, which is then used to establish a bypass-path model of the 25 Hz phase-sensitive track circuit and derive the corresponding MTL equations. Based on the principle of transformer mutual-inductance circuits, the voltage and current relationships of IBs at the sending and receiving ends of the broken rail section are analyzed. The boundary-condition parameter matrix of IB is derived, and the longitudinal distribution of voltage and current under broken rail conditions of the 25 Hz phase-sensitive track circuit is obtained. A decoupling algorithm based on the IB boundary-condition is proposed. The bypass-path model and the decoupling algorithm are validated through laboratory and field tests. Considering that the receiving voltage in the bypass path is non-zero under broken rail conditions, the effects of the IB connection scheme, break location, ballast leakage, and cross-bond distance on the receiving voltage are investigated. The results show that, for sections with IBs fully connected, the receiving voltage increases as the break location approaches the mid-section and as the ballast resistance increases. For sections with the sending-end or receiving-end IB disconnected, the receiving voltage increases as the break location approaches the end where the IB remains connected, and it first rises and then falls as the ballast resistance increases. When the cross-bond distance exceeds 2 km, the receiving voltage becomes nearly invariant, and 2 km can be used as a reference value. A higher receiving voltage under broken rail conditions makes broken rail detection more difficult. Therefore, it is recommended that, for sections with fully connected IBs, broken rail detection be tested using a criterion of a 40% drop in receiving voltage, whereas for sections with the sending-end or receiving-end IB connection disconnected, broken rail detection be tested by removing the single-rail connecting wire at the IB-connected end.