The invention discloses an open window-hidden curtain wall system which is formed by splicing a plurality of curtain wall window units, wherein each curtain wall window unit comprises two columns, two beams and an open fan; the two columns and the two beams enclose a curtain wall frame provided with a through opening; the open fan covers/opens the through opening, wherein a groove is formed in an abutting position between the curtain wall frame and the open fan; the groove is communicated with the through opening; when the open fan is located at a closed position, the open fan is accommodated in the groove; each curtain wall window unit further comprises a plurality of decoration plates; one decoration plate is arranged at the abutting position between an outer surface wall of the curtain wall frame and the open fan; the decoration plate comprises a first decoration plate and a second decoration plate adapted to the first decoration plate; the first decoration plates are connected around the open fan; the second decoration plates are connected to the curtain wall frame; when the open fan is located at the closed position, the first decoration plates prop against the second decoration plates and flush with the outer surface wall. The open window-hidden curtain wall system provided by the invention saves materials, and is reasonable in structure and large in ventilation and lighting area.