Abstract: The turn-off characteristics of converter valve thyristor will change with outer circuit condition dynamically such as forward current, current zero-crossing declined rate and thyristor junction, when an AC fault occurs at the inverter side of a line-commutated converter-based high voltage direct current (LCC-HVDC) system. However, regarding the thyristor as an ideal switching device or only remaining fixed margin for it as most research may cause inaccuracy of commutation failure judgement. To clarify the influence of variation of thyristor turn-off characteristics on precision of the commutation failure judgement, this paper analyzes how thyristor turn-off characteristics affect commutation process in theory. Then, it proposes and builds a converter dynamic turn-off model with proved correctness by comparing with the thyristor's detailed physical model in SABER. Finally, the simulation results of the converter dynamic turn-off model with the traditional model under the single and three-phase earth faults in PSCAD/EMTDC are compared separately. The results show that using a fixed turn-off margin model is considered as preventing commutation failures, which could lead to high turn-off demands on the system. However, in certain scenarios, it may also result in inaccuracies in the timing of commutation failures. By adopting the proposed converter dynamic model, the prediction of commutation failures can be made more precise.