A self-destruct-upon-opening cap applicable to a small-diameter connector used for a gastrointestinal tract includes a top lid and a cap body. The top lid is arranged on the cap body and fixedly connected to the cap body, and a connecting structure is arranged at one end of the cap body which is not connected to the top lid. The cap body includes a first casing and a second casing. One side of the second casing which is not provided with the connecting structure extends into the first casing, and a top face of the second casing is provided with serrated protrusions in a circumferential direction. The side of the second casing which is not provided with the connecting structure is fixedly connected to the first casing through ribs or contact points. An inner side face of the top lid is provided with grooves matching with the serrated protrusions.