In dynamic wireless power transfer systems for smart rail trains, a high fluctuation rate of mutual inductance leads to reduced efficiency when an offset occurs between the transmitting and receiving coils. This paper explores the mutual inductance characteristics and the variation rule of magnetic induction strength between the transmitting coil and the receiving coil at offset. And proposes an I-shape coil structure. The I-shape coil structure has good anti-offset performance in the direction of motion, and the maximum offset distance is up to 1.2 times the outer length of the transmitting coil. First, the mutual inductance characteristics of this I-shape coil structure are investigated based on the coupling mechanism of the I-coil. Meanwhile, a mutual inductance optimization method is proposed, which is used to obtain the values of each coil parameter that satisfy the requirements. Secondly, the magnetic core optimization of the I-coil structure has been carried out to achieve higher mutual inductance and better transmission efficiency. Finally, a wireless power transfer system is constructed based on the obtained coil and magnetic core parameters. Simulation and experimental tests are carried out for this coil structure and the coil structure with magnetic core, respectively. The experimental results verify the rationality and correctness of the structure. The results show that the maximum mutual inductance fluctuation rate is only 4.97% in the coil structure without magnetic cores with the offset distance between the transmitting and receiving coils at 120% of the outer edge length of the transmitting coil. With the addition of the magnetic core, the maximum mutual inductance fluctuation is only 5.02% with an efficiency of 97.61% at an offset distance between the transmitting and receiving coils of 120% (50.8 cm) of the outer edge length of the transmitting coil.