With the continued development of hybrid AC/DC power grids, the practice of having AC and DC transmission lines on the same tower has become increasingly common. Consequently, AC/DC cross-line faults, which are unique to this configuration, are expected to occur more frequently. Focusing on ungrounded AC/DC cross-line faults, this paper analyzes their fault characteristics and investigates the adaptability of AC line distance protection under such conditions. The results indicate that after the fault occurs, the measured impedance of the faulted phase lies in the fourth quadrant of the complex plane, exhibiting capacitive characteristics and having a magnitude larger than the total line impedance. This causes a reduction in the sensitivity of the distance protection, leading to a risk of protection failure to trip. A fault phase selection criterion based on the difference in the three-phase AC current waveforms is proposed. Furthermore, a method for determining AC/DC cross-line faults based on measured impedance properties is developed, achieving both fault phase selection and fault discrimination. In addition, a same tower AC/DC transmission line model is built on the PSCAD/EMTDC simulation platform, and extensive simulation experiments verify the effectiveness of the proposed fault discrimination method.