Silica nanospheres have attracted tremendous interest due to their importance in extensive applications. However, the direct large-scale fabrication of silica nanospheres with controlled morphology and high purity remains a significant challenge. In this work, silica nano-/submicron spheres were successfully synthesized by a simple method through pyrolysis of an amorphous polysilazane preceramic powder with catalyst FeCl2. The synthesized spheres possess well-designed shape with diameter of 600–800 nm and high purity. The surfaces of the spheres are smooth and clean without any flaws. Besides, the spheres are identified as amorphous silica, and their growth mechanism was also proposed.