Abstract: Establishing the fixed-admittance model of power electronic devices is one of the main ways to realize small-step real-time simulation of large-scale power electronic systems. This paper focuses on the simulation accuracy of fixed-admittance model at high switching frequency. Firstly, the virtual power loss of fixed-admittance model is analyzed, and the influence of numerical integration method on the loss is quantified. Secondly, an additional-branches fixed- admittance model is proposed aiming at the structure of insulated gate bipolar transistor (IGBT) with anti-parallel diode, which changed the structure of the additional branches with the switching state, so that it could still ensure low power loss without interpolation. Thirdly, for the typical application scenarios-three-phase two-level voltage source converter (VSC), the losses of the proposed model with interpolation and without interpolation are quantitatively calculated and qualitatively analyzed separately. Finally, the applicability of the proposed model and the limitation of the equivalent interpolation are verified based on PSCAD/EMTDC.