According to the purity requirement for Si powder, a cold plasma reaction chamber with long drop distance and high reaction rate is provided in the paper. In the selection of technology parameter, we put stress on increasing the thickness of sheath, raising the densities of reaction particles, and controlling the speed of pump. The purifying rate, collective ratio, and drop time of Si powder were raised effectively as a consequence of these selections. The experimental results show that solar grade Si (4N) can be gotten from industrial Si (Melatturgical grade Si-2N) in the plasma system.