The EMTDC/PSCAD simulation can be used to study the closed-loop transfer characteristics of HVDC system, but it is very difficult to understand analytically the transfer characteristics of each system element, i.e. transfer characteristics of valves of HVDC system. Thus, the model based on the modulation theory has been widely used to describe the open-loop valves transfer characteristics of thyristor-based convertors. However, it is found that there is discrepancy between the EMDC/PSCAD simulations and the open-loop valves transfer model based on the modulation theory in subsynchronous oscillation analysis. It is further revealed that when employing the model based on the modulation theory, the assumption that the valves conduct at equal time intervals is no longer true in low and subsynchronous frequency band. Hence, in this paper, a new model is proposed to describe the valves transfer characteristics of the thyristor-based HVDC convertor, which helps understand the behavior of valves conducting at unequal time intervals. An EMTDC/PSCAD-based simulation system is employed to verify the new model and compare the results of the new model with that of the model based on the modulation theory in the low and subsynchronous frequency band. The new model is useful to understand how the additional spectral components are generated by the HVDC system considering the transfer characteristics of the converter in low and subsynchronous frequency band.