The concern of the world about Building Integrated Photovoltaic that can alternate with the exterior materials in the building is growing from day to day. There are many kinds of the solar photovoltaic which can apply to the building. Among them, a-si thin film solar cell is possible to alternate the window and to daylighting. So, if alternating the glass material that occupy the great part of building facade, growth possibility and the effect from BIPV will be maximized. Therefore, in this study, the database as to optical performance depending on the spectrum section that is the most base step was established for applying a-si BIPV window system in the building using thin film solar cell, And optical and thermal performance of a-si BIPV window system were evaluated by simulation. For conducting this, the optical properties of PV raw module and PV single module that are Tsol(0.086, 0.067 each) and Tvis(0.091, 0.082 each), SHGC(0.303, 0.295 each) were evaluated. And we analyzed thermal performance of a-si BIPV window system by the configuration of window components and we suggested the most suitable configuration methode of a-si BIPV window. Each of the U-factor of Double and Triple a-si BIPV window are 1.174$W/m^2{cdot}^{circ}C$, 0.863$W/m^2{cdot}^{circ}C$. As a result, optical properties of a-si BIPV window system has low transmittance comparing common window system but it can meet the illumination depending on the building use and reduce the glare effect that can occur when clear window system is used if a-si BIPV window system is applied on a large area in building facade.