In this paper, a 3-floors building with capacious spaces was introduced. It has unique shape and all-glass envelope, which worsens the thermotechnical performance of the envelope, deteriorates the indoor thermal comfort. By simulation, the indoor thermal environment of this building can be obtained. Standard Effective Temperature (SET) was adopted to assess the thermal environment of the occupied zones in the building. Three cases were assessed and compared: the original design, the design with some ceiling sunshades and the design with some ceiling and sidewall sunshades. From the comparison, the appropriate building envelope design and air-conditioning system design schemes which satisfy human thermal comfort requirement were obtained. From the case it can be found that, in the architectural design, the application of building simulation takes an important assistant effect. Sometimes this application may be indispensable.