A rotary structure having a large upper part, a small middle part and a large lower part is formed between guiding blocks (11) of a guiding mechanism (1). A positioning groove (10) is disposed at the wasp-waist of the guiding block (11). The bottom of a sealing ring (3) is embedded in the positioning groove (10) to surround the guiding block (11). When a surgical instrument is inserted, the sealing ring (3) confined within the positioning groove (10) extends out of the gap between the guiding blocks (11) to surround the instrument. The contact area between the sealing ring (3) and the instrument provides a guiding function to different forms of surgical instrument and transforms a “soft-hard” friction area between the sealing ring (3) and the instrument into a “hard-hard” friction area between a plastic guiding surface of the guiding blocks (11) and an outer metal sheath of the instrument.