The invention provides a packer comprising a compression rubber sleeve and an expansion rubber sleeve arranged up-down outside a central pipe; the compression rubber sleeve can realize setting when fluid pressure rises in an oil pipe, and a holding structure is arranged so as to keep the setting state; the expansion rubber sleeve can realize setting when a flush fluid is injected into a hohlraum; the well flushing work can be carried out after the compression rubber sleeve is set and the flush fluid is injected into the hohlraum, so the flush fluid cannot enter an oil reservoir so as to protect the oil reservoir, and dual-setting can provide more reliable setting effect; in addition, the special hohlraum and unlocking structure are designed, and the holding structure can be unlocked when the flush fluid enters the hohlraum; once the flush fluid stops injecting, the compression rubber sleeve and expansion rubber sleeve can automatically rebound, i.e., the packer can be automatically unsealed when the well flushing is finished; extra operations like pulling up an oil pumping string are not needed, thus ensuring normal production of the oil well with no interruption while finishing the well flushing; the packer can be repeatedly used underground.