The invention belongs to the technical field of laboratory rat catching devices, and in particular relates to a length-retractable metal clamp. The metal clamp comprises a left clamp body and a rightclamp body. Each clamp body comprises a handle ring, a sleeve is arranged at the lower end of each handle ring, and the lower side wall of each sleeve is provided with a first threaded hole which is internally provided with a fixing screw. A telescopic rod is arranged below each sleeve, and each sleeve sleeves the upper part of one telescopic rod. The upper part of each telescopic rod is providedwith a first blind hole matched with one first threaded hole. A first groove is formed in the edge of the upper end surface of each telescopic rod, and the length of each first groove is smaller thanthe length of one telescopic rod. A second threaded hole is radially formed in the lower part of each telescopic rod, and a ball plunger is arranged in each second threaded hole. A casing pipe is arranged below each telescopic rod, a second blind hole matched with a third threaded hole is arranged on the upper part of each boss, a second through hole is arranged in the front end surface of each clamp arm, the two clamp arms are hinged together through the second through holes, and the lower part of each clamp arm is provided with a clamp head. The purpose of the metal clamp is to solve the problem of inconvenient adjustment of the length of clamp arms caused by defects of an existing rat catching clamp.