Using FFC-Cambridge Process to prepare Si from SiO2 is a promising method to prepare nanostructured and highly pure silicon for solar cells. However, the method still has many problems unsolved and the controlling effect of the cell voltage on silicon product is not clear. Here we report in this article that nano cluster-like silicon product with purity of 99.95% has been prepared by complete conversion of raw material SiO2, quartz glass plate, using constant cell voltage electrolysis FFC-Cambridge Process. By analysis of XRD, EDS, TEM, HRTEM and ICP-AES as well as the discussion from the thermodynamics calculation, the morphology and components of the product based on the change of cell voltage are clarified. It is clear that pure silicon could be prepared at the cell voltage of 1.7–2.1 V in this reaction system. The silicon material have cluster-like structure which are made of silicon nanoparticles in 20–100 nm size. Interestingly, the cluster-like nano structure of the silicon can be tuned by the used cell voltage. The purity, yield and the energy cost of silicon product prepared at the optimized cell voltage are discussed. The purity of the silicon product could be further improved, hence this method is promising for the preparation of solar grade silicon in future.