To accurately study the fault transient characteristics of a photovoltaic system, given the random and intermittent characteristics of a photovoltaic power generation system, the transient process of different faults on the tie-line of a photovoltaic system is analyzed. For power system voltage symmetrical and asymmetrical dropping, different control strategies are proposed under different circumstances. Then the corresponding control targets are obtained. Based on the proposed control strategies and the established photovoltaic models, aiming at different faults, we simulate the transient process of power, voltage and current from the photovoltaic side. The simulation results demonstrate that the transient process is really influenced by the control strategy and quite different from traditional generators. This has important practical and theoretical significance on tie-line protection, as well as the incorporation of photovoltaics in the power grid. This work is supported by the Science and Technology Project of the Headquarters of State Grid Corporation of China (No. 52100218000U).