The utility model relates to an associated gas recovery process for crude oil extraction in an oil field, in particular to a casing gas recovery device for an oil well in oil field. The device is characterized by the inclusion of a synchronous rotating compressor, crude oil from a wellhead, casing gas from the wellhead, valve pipelines and an oil pumping unit, wherein the crude oil from the wellhead is in pipeline connection with an inlet port of the synchronous rotating compressor through a first pipeline provided with a throttle valve, a globe valve and a filter; the casing head gas from the wellhead is in pipeline connection with another inlet port of the synchronous rotating compressor through a second pipeline provided with a throttle valve, a globe valve and a filter; and the compression outlet end of the synchronous rotating compressor is communicated with a downstream pipeline through an output pipeline connected in series with a check valve, a buffer tank and an outlet globe valve. The device provided by the utility model can effectively reduce wellhead back pressure, improve oil extraction working condition, and reduce the energy consumption of the oil pumping unit.