Line commutated converter based high voltage direct current (LCC-HVDC) is the key to large-scale grid connection and long-distance transmission of renewable energy. However, faults such as DC blocking and commutation failure may lead to short-term excess reactive power and transient overvoltage at the sending end, endangering operational safety. In this paper, a method based on grid-forming based reactive power compensation device (GFM-RPC) to suppress transient overvoltage at the renewable energy transmission terminal is proposed, which is different from the traditional reactive power compensation with the characteristic of current source based on voltage-current cascade control. A voltage dynamic analysis model based on differential-algebraic relationship is constructed to clarify the mechanism of GFM-RPC suppressing transient overvoltage, and the advantages of the proposed method compared to existing methods based on static synchronous compensator (STATCOM) for suppressing transient overvoltage is compared and analyzed. The simulation is used to verify the effect of GFM-RPC on the suppression of transient overvoltage at the renewable energy transmission terminal, and the influence of the main parameters on the overvoltage suppression effect is analyzed. It is shown that reactive power compensation devices such as STATCOM with external characteristics of current sources exhibit a reverse regulation characteristic of deteriorating voltage dynamics at the moment of DC transmission system fault, while the reverse regulation characteristic can be eliminated by GFM-RPC, as well as suppression of voltage magnitude overshoot can be achieved through reasonable parameter configuration for GFM-RPC.