With the growing demand for flexible interconnection in distribution networks, thyristor-controlled series-parallel medium-voltage distribution transformers with soft interconnection and power flow regulation capabilities have attracted increasing attention. However, existing phasor-based models struggle to accurately characterize the transient behavior of such devices during processes such as tap switching. To address this issue, this paper proposes a transient model suitable for this type of equipment in medium voltage distribution networks. First, based on the device topology and control strategy, a transient mathematical model is derived to clarify the relationship between the conduction states of the thyristors in the secondary circuit and the corresponding voltages and currents. Subsequently, a component level simulation model is constructed in PSCAD to verify the time-domain response under different operating conditions. Finally, comparative analysis is conducted using on-site operation data. The results indicate that the established model can accurately reproduce the transient characteristics during tap switching, with good agreement between simulated waveforms and measured data. The model provides methodology and simulation support for transient analysis, fault mechanism studies, and protection strategy design in systems incorporating this device.