The present disclosure discloses an array substrate and a preparation method thereof. After a first passivation layer is formed, residual gas is directly drawn out of a closed chamber to prevent the residual gas from reacting to form an unstable layer on the first passivation layer. Furthermore, after the residual gas is drawn out, a preset gas fills the closed chamber, and is retained for a preset time period and then drawn out. The retaining of the preset gas can effectively alleviate the damage to the passivation layer by static electricity.