In the purpose of improving indoor comfort and achieving building energy conservation, significant efforts have been made to improve the performance of window systems. Smart windows have been increasingly considered as an efficient technology with adjustable control of their thermal and/or optical properties in response to instant changes of the environment. This paper explores the potential of using a durable Mg-Y based switchable mirror gasochromic (GC) material for building window applications. The selected Mg-Y based GC window provides a degree of flexibility to reflect the undesirable incident solar radiation rather than the other types of GC windows, which absorb the solar heat that might eventually go into the indoor space through convective, conductive and radiative heat transfer. Building simulations were carried out for a typical office with Mg-Y GC window applied using EnergyPlus. The characterization of the selected gasochromic smart window and its impact on building performance was comprehensively studied under 5 diverse climates in China. In addition, various control strategies in response to different environmental conditions or indoor comfort criteria are also considered. The results indicated that Mg-Y based films with excessively low transmittance at the reflective state are not beneficial from the perspective of building energy conservation, while potentially developed Mg-Y windows with relative higher transmittance and larger transmittance modulation can yield energy conservation of up to 27% when compared with standard double glazing. Overall, the work presented in this paper may be seen as offering potential advice and guidance on further development of switchable mirror GC material that seek to be applied into building windows for amplifying improvements in energy efficiency and occupant comfort.