In this paper, signal integrity (SI) of a mobile high-definition link (MHL) system is analyzed, having the discontinuous and asymmetric structures. In the SI analysis of the MHL system, three dimensional (3D) electromagnetic (EM) solver and time domain reflectometry (TDR) method are carried out. As a result, the differential signals with the discontinuous reference plane experience the degeneration of transmission characteristic due to the impedance mismatch, rendering by discontinuities in return current path of the signals. Moreover, it is found that the SI characteristic of the differential signal could be significantly degenerated, when the asymmetric guard traces with ground vias for electromagnetic interference (EMI) reduction are located closer to the signal trace than the balanced reference plane. Therefore, SI can be guaranteed with widely open eye diagram using the symmetric and continuous reference plane for the coupled signal lines in the high-speed digital interconnect.