The second generation of high temperature superconducting (HTS) strip has the outstanding characteristics of high critical temperature, high load current density and high critical magnetic field, so the HTS coils wound by the strip have broad application prospects in the field of electric power, such as magnetic energy storage system, high speed maglev train and wind power generation system. In order to promote the further development of HTS magnetic energy storage system (SMES), this paper conducts the modelling and simulation research on the ac losses of a 100- MJ toroid-type HTS SMES. Firstly, the geometric structure of the system are introduced. Then, the ac loss models of HTS coils and the non-superconducting components in the energy storage system are established by H-formulation and A-formulation, respectively. Based on both the models, we calculate the ac losses of HTS coils and related components during the process of energy storage and release. Finally, the model coils are winded and impregnated to show the structure.