The diffusion of proton in semiconductor electronic devices can cause performance degrading. Nevertheless, it is seldom known to date about the mechanism of this process in oxide. In this work, proton diffusion is studied in a-SiO2 using Ab Initio Molecular Dynamics(AIMD) from the atomic scale. The diffusivity, activation energy in a-SiO2 are calculated, and the diffusion paths are discovered. In a-SiO2, the proton is found to be diffusing via forming and dissociating bonds with oxygen atoms. Two diffusion means of proton in a-SiO2 are proposed, hopping diffusion and rotation diffusion. Meanwhile, the energy barriers for these two diffusion methods are calculated respectively.