The invention relates to an atmospheric pressure diffusing discharge device and a method for depositing a SiO2-like thin film on a metal surface. The atmospheric pressure diffusing discharge device comprises a pin electrode, a metal base and a gas mixing chamber, wherein the pin electrode is vertically placed, and the top end of the pin electrode is connected to a high-voltage pulse power source; the metal base is arranged below the pin electrode and is grounded; the gas mixing chamber is arranged above one end of the metal base, and a gas spraying port is formed in one side of the gas mixing chamber and faces a deposition area; the gas mixing chamber is in the shape of a cylinder or a cube and is made from an insulating material; and the gas mixing chamber is connected with a gas bubbling device through a mixed gas outputting pipe. The atmospheric pressure diffusing discharge device is simple and convenient to operate, discharge plasmas which are wide in range and uniform can be obtained on the metal surface under the relatively simple conditions of room temperature, atmospheric environment and the like, and the defects that due to a traditional discharge mode, the thin film deposition range is small and uniformity is poor are overcome.