Si-Ge alloys are a new type of semiconductor material that are of great significance for the development of microelectronic technology, especially in the photoelectricity industry and for thermoelectric conversion in high temperature applications (>700 K). In the present work, a novel Ge allotrope in the C2/c phase with band gap of 1.102 eV was firstly proposed, which is suitable for the absorption of sunlight. C2/c-Ge are mechanically, dynamically and thermodynamically stable. A series of Si24-xGex alloys (x = 0, 8, 16, 24) in the C2/c phase with band gaps of 1.10-1.50 eV are predicted by ab initio calculations at ambient conditions. The Si24-xGex alloys (x = 0, 8, 16, 24) in C2/c phase have better absorption ability than that of the Si in diamond, hP12 and oC12 phases. The Si24-xGex alloys in the C2/c phase have the strong absorption in the visible, which have a great impact on the new-generation photovoltaic applications.