The utility model provides a packer capable of being automatically expanded in oil and water. The packer comprises a base pipe, an automatic expanding rubber barrel and two ring assemblies; the automatic expanding rubber barrel and the ring assemblies respectively sleeve the base pipe; the two ring assemblies are correspondingly adjacent to two ends of the base pipe; each ring assembly comprises an end ring, and a limiting ring which is prepared from rubber; the end rings are fixed to the base pipe; the end part of the automatic expanding rubber pipe are reduced to form an outer taper surface; the inner circumferential surface of each limiting ring is an inner taper surface which is matched with the outer taper surface; the limiting rings are correspondingly arranged on the outer taper surface of the automatic expanding rubber barrel through the inner taper surface by a sleeving manner; the end rings axially extend opposite to the limiting rings to form a ring lug which is correspondingly pressed onto each limiting ring. According to the packer, the ring assemblies are arranged at two ends of the automatic expanding rubber barrel, which limit the axial expanding of the automatic expanding rubber barrel during working, and therefore, good sealing effect is realized, and the shortcomings of the prior art are overcome.