The utility model discloses an automatic casing air discharge device for preventing oil blowout used in oil well in petroleum industry. The automatic sleeve air discharge device is applicable to oil wells with high casing pressure, and can be used to control the liquid level of a well casing within a rational submergence depth in an oil well so as to prevent occurrence of gas lock and to ensure normal operation of an oil well. The automatic casing air discharge device comprises a pressure gauge, an automatic air discharge mechanism and an oil blowout prevention mechanism, wherein the automatic air discharge mechanism is arranged on an upper casing, the oil blowout prevention mechanism is arranged on a lower casing, a clasp joint is arranged on the lower casing, and the pressure gauge is installed on the clasp joint and the connecting pipe of the casing. Automatic casing air discharge function can be realized through via the automatic air discharge mechanism base on preset pressure, thereby preventing blowout of crude oil, and environmental pollution and obstruction of gas pipes during routine casing air discharge, and reducing labor intensity.