The participation of photovoltaic (PV) plants in supporting the transient voltage caused by commutation failure in the line-commutated-converter-based high voltage direct current (LCC-HVDC) system is of great significance, as it can enhance the DC transmission ability. However, it is found that the grid-following (GFL) PV converters face the problem of mismatch between reactive power response and transient voltage characteristic when the voltage converts from low voltage to overvoltage, further aggravating the overvoltage amplitude. Thus, this article proposes a transient voltage support strategy based on the grid-forming (GFM) medium voltage PV converter. The proposed strategy takes the advantage of the close equivalent electrical distance between the converter and grid, which can autonomously control the converter terminal voltage through GFM control with adaptive voltage droop coefficient. The simulation results show that the proposed strategy can ensure the output reactive power of the PV converter quickly matches the transient voltage characteristic at different stages, indicating that the proposed strategy can effectively support the transient voltage.